Questions and Answers: It’s Your Turn

17 Dec 21 56 Comments

A Fear of Landing reader asked me a great question in email.

Is there some way of asking general technical questions that relate to aircraft or airport operations rather than a specific post. One of your posts discussed runway naming, but I’ve got more questions about this. This particular example could be answered by yourself as a pilot, but other questions might be better as a general discussion.

I answered that no, there wasn’t really a way of doing this but I think it is a great idea, so there is now! I will do an irregular feature in which the community can come together and ask questions and answer questions without it needing to be about a specific post.

If you have an aviation question that you would like to ask, please post in the comments. If you aren’t already cleared to comment, be patient and I promise I will get the question up as soon as possible. If you are reading the comments and you know the answer to one, please take the time to hit “reply” to the comment. This means that the answer is threaded to be clearly under the question so it is easy for people to follow. You do not need to be a pilot or even in the aviation industry to answer a question if you know the answer… but do try to stick to what you are confident about, rather than guessing.

Number one rule: There are no stupid questions.

I think this will be a lovely way of sharing aviation information and if it works, I will pick up any unanswered questions from this post and use them to seed a new edition of Question and Answers in the future. So don’t be afraid to leave a question even if the conversation seems to have moved on.

Let’s start this off with a few questions I already have from people in the comments and in my email.

Gordon: We have helicopters flying overhead reasonably often, and sometimes they make a real thumping or beating sound, like they are struggling to move. We can hear all helicopters coming due to the engine sound and low flying, but these thumpers really stand out. What causes this?

Mike: When talking to ATC on approach, what is a let-down service?

Louis: I’m making a model of a Pitts Special, using Betty Skelton’s Lil Stinker for my design. I haven’t found any images that how the bottom of the aircraft: does anyone know what the paint scheme of the bottom of Lil Stinker looked like?

Gordon: How do pilots deal with flying west in the late afternoon with the sun in their eyes?

John: Why are airport maps called charts?

Robert: When are you coming out with a new book?

OK, maybe you all should not answer that last one (Answer: hopefully in March!).

But please, take a stab at answering the others or leaving your own questions in the comments for people to ask.

Category: Demystifying,


  • In aviation maps are called charts because at the beginning of aviation lighter than air craft borrowed naval terminology. Terms like airSHIP, rudder, and cockpit are all naval terms. This practice continued with heavier than air craft.

    • Makes sense, but then I have to wonder why charts in the first place or whether there’s any qualitative difference between charts and maps.

      • Charts of the sea and the air show things that are not really there – like shipping lanes in the sea, airspace boundaries etc. Maps are principally of the land and show physical features like structures, roads, coastlines. The other significant difference, and what really sets a chart apart from a map is that a map depicts what is on the surface of the earth, whereas a chart depicts information in a third dimension of height or depth: ABOVE the surface (aviation chart) or BELOW the surface of the sea (sea depths on a nautical chart).

    • One of the main differences between a map and a chart is that the chart has a high degree of accuracy for measuring the distance between any two features. Also, navigational charts frequently have longitude and latitude lines on them. The latitude lines have 60 hatch marks between each degree. The hatch marks represent one “minute” of a degree. A minute of a degree of latitude is a nautical mile! Sextants are calibrated in degrees and minutes. An outcome of celestial navigation on the water and in the air was the use of nautical miles per hour for speed. (~ 50 years ago I was a navigator on a B-52 bomber doing celestial navigation )

  • Generally speaking, the ‘thump-thump-thump’ (or wap-wap-wap) of a helicopter is due to the rotor blade passing over the tailboom. Helicopters with fewer and relatively larger blades (think Huey) will have a more emphatic thump.

    That said, I’m not particularly expert about this – perhaps certain helicopters are just designed to make less noise when that happens?


    • I’m really interested in the answer to “what’s ‘let-down’ service?” too!

      I googled ‘atc “let-down” service’ and was surprised to get absolutely nothing relevant.

      • Looks like that one is my fault. :(

        In the context of Air New Zealand flight 901, there was a radar-controlled letdown at McMurdo.

        The US Navy Air Traffic Control Centre (ATCC) “Mac Centre” suggested that the aircraft crew take advantage of the surveillance radar to let down to 1500 feet during the aircraft’s approach to McMurdo and the crew indicated their acceptance of this offer.

        But thinking of this in terms of the service that ATC were offering, I referred to this as a let-down service as opposed to a radar service for a let-down procedure.

        From Journal of Navigation:

        The purpose of an instrument let-down is to bring an aircraft past all hills and obstructions to a particular position at low height. If the pilot follows the correct procedure, the aircraft will obviously clear the hills and obstructions. But owing to various interruptions and distractions, difficulties of reception or the need to give partial attention to other duties, he is often unable to follow the procedure exactly; he will then want to know whether departure from the procedure requires immediate rectification, or whether he is at liberty to return gradually to the correct path. A badly working radio aid, or some other difficulty, may make it necessary to abandon a partly completed approach; he will then want to know the best way to extricate his aircraft from a possibly dangerous situation. Furthermore, the pilot often has to improvise an ‘intermediate procedure’, to connect between his arrival over the aerodrome and the beginning of the let-down. For these reasons he needs to know the positions of the hills and obstructions in relation to the let-down; and the only way to present this information to him quickly and unambiguously is by making hills and obstructions the principal features of let-down charts. The local knowledge possessed by pilots of scheduled services does not lessen the need for an effective presentation of this feature on the charts, as it is precisely when landing at unfamiliar aerodromes that the charts will be most used.

      • In its simplest form a let-down is a procedure to get an aircraft safely to a lower height/altitude. An ATC unit will have surveyed an area around an airport for significant obstacles, from that they calculate a safe altitude for a particular segment and for the area as a whole, these minimum altitudes/heights are published in charts for pilots to know.

        In IMC, a pilot can follow the charts to let-down either to the runway or to just get below cloud. However, ATC can also offer a radar letdown to transiting or arriving aircraft to get them into VMC from where they can make a visual approach or continue en route in VMC. In my RAF ATC days, we used to carry out let-down procedures using CADF (a little green worm like display that gave the direction of the aircraft from the airfield receiver when the pilot transmitted on his radio), this procedure was ATC controlled using nothing more than the CADF and knowledge of the local terrain and usually consisted of homing to the overhead, followed by descending in teardrop pattern usually 15 degrees off the runway heading turning onto the extended centreline at half the starting height + 1000ft. This was the most common way of recovering single-seater jets before they were equipped with ILS.

        Hope this helps.

    • Again I am not a helicopter pilot but I have noticed that twin rotors of the Chinook make far more noise than others, obviously due to buffeting of the airflow as the blades of the rotors overlap. Maybe this is what was heard?

      • Yes. The relative speed of the rotor blades is doubled, because one’s going the other way, compared to a stationary tailboom – so you get a bigger ‘whap’.

        This is a problem, actually, with all contra-rotating (and close together) rotors. Some aircraft with two counter-rotating propellors on the same nacelle are known to be fantastically noisy (Russian TU-8 Bear, US “Thunderscreech”, although there’s other reasons, too).

        I’m not a licensed helicopter pilot, although I’ve piloted some, under the careful watch of an instructor.

        The comment below is also correct; A heavily-loaded helicopter, or one climbing, will make more rotor noise because the rotor is just trying to move more air. Adjusting the collective changes the angle of attack of the blades – more angle, more lift, more air moved, more noise.

        • “The relative speed of the rotor blades is doubled, because one’s going the other way, compared to a stationary tailboom…”

          Eh? In the area where the rotors of a Chinook overlap, the blades are going in the same direction, like meshing gears.

          Of course, you are going to have volumes of air moving in different directions with loads of turbulent flow, which makes them noisy. (Think a hand whisk with two whisk heads.)

    • The helicopter pilot controls the pitch of the blades with his left hand using the “Collective”. Changes in collective are generally used for changes in altitude. A change in the noise from the blades is probably the pilot adjusting his altitude.

    • In answer to Gordon’s question about the noise generated by rotary wing aircraft may I submit a response.

      Helicopters fly simply by the brute force of the engine(s) that keep the rotor disc spinning and get very little lift from the fuselage passing through the air.  Climbing and manoeuvring of any sort require additional power to be applied through a careful mix of engine revs and alterations of rotor blade angle via the collective lever.

      Staggering amounts of energy are expended, especially in the hover, some of which ends up being dissipated as noise.  Take the case of a Chinook, which can have a maximum gross weight of 50,000 lbs.  Hardly surprising that the level of noise is so high.

      • Actually, helicopters also get quite a lot of lift from the ‘rotor disc’ – If flying at some horizontal speed. It’s a complicated question of fluid dynamics as to why, but suffice it to say – they do.


  • Sometimes we are asked a question about a subject that we are familiar with, but when it comes to finding an answer we realise that it is not as easy as we thought.
    Map vs, chart:
    Chris came with a very good answer, yes I buy that. But I think that there is a bit, actually a good bit more to it. A map is just a lay-out. A chart must conform to certain standards because it is used for specific purposes, such as navigation.
    And so it must display information that is of use, of critical use even, to the specific category of professionals who need it. The scale and accuracy must be of as high a standard as possible. And of course, when we use it for navigation at sea it must show depths, obstacles like sunken wrecks, markers, buoys. Not so important for air navigation – unless we operate a seaplane of course.
    The projection used, Lambert, conical, Mercator etc. is of vital importance because if determines the course plotted, the track flown and enables the pilot (or navigator) to determine whether the craft is on track.
    Airport charts, like in the Jeppesen, display very important information about aprons, taxi routes, departure routes (SIDs) and arrivals (STARs) as well as the various approach procedures. Again, they contain very specific and highly detailed information for pilots.
    I think that this is more colloquial, rather than an official term. ATC will give instructions to arriving aircraft about their descent, I suspect that this applies more to the segment known as “intermediate approach” where ATC will guide an aircraft through various levels or altitudes for the final approach..

      • As a retired RAF fast-jet pilot, a let down service (rather than an actual procedure) was common. It’s usually given on request typically by ATC, using radar, to enable aircraft to get VMC below clouds safely avoiding terrain, other aircraft, controlled airspace, obstacles and so on. Less common these days where aircraft navigation is super accurate and they have their own radar and they can do it for themselves (although still handy for aircraft deconfliction). It’s not exclusively for use by the military by the way and can be particularly helpful for instrument rated light aircraft pilots. By the way, ATC would only give a service down to minimum safe altitude – that is the published safety altitude for that location on the regional QNH (altimeter pressure setting). Further descent beyond that point was entirely up to the pilot.

  • Hi Sylvia, You replied by repeating the question. It is true that I have been removed from aviation for a long time now and perhaps my memory is a bit hazy. But, I repeat now, I have never heard about a “let-down procedure”. Not as an official, published term. On the other hand, the international aviation language is English. The majority of the people who use “aviation English” in the course of their daily work are not native English speakers, and even if, there are differences between, for argument’s sake, an English-language speaker from the UK, the USA, Australia, South Africa, New Zealand, Ireland.
    “Letting someone down”, in colloquial parlance, has a different meaning than “talking an aircraft down” through the various stages of descent and approach of an aircraft. So I am convinced that a “Let-down procedure” is not an official ICAO-sanctioned term.
    But if someone can give a more accurate definition, I am sure that we would like to hear (read) it!

    Meanwhile: I wish Sylvia and all her readers a very happy Christmas. If you happen to live in a country that does not officially celebrate it, in my heart you are all part of our big brother- or sisterhood no matter your race, nationality or creed. So feel welcome to celebrate with us.

    • Happy Solstice to you too. We’re all on the same planet here, and the sooner we figure that out, the happier we all shall be. ‘Til then! J.

  • @mike I found a document from Bureau of Land Management (BLM) National Aviation Plan (NAP) that describes it thus: “Cargo Letdown is a procedure used to lower cargo out of a hovering helicopter to the ground with the use of a lowering line and rappel anchor. This procedure is used by helitack programs to get needed equipment and supplies to the ground when conventional methods are not the most efficient option.”

    I found another that referred to a “nylon letdown”, which turns out to be a parachute! I seem to have lost that doc though.

  • Hi Sylvia, I have always wanted a serious answer to this question: Where would an ETOPs certified twin-engined passenger aircraft flying across the sea from Johannesburg, South Africa, to Perth, Australia, divert to in an emergency? I see no suitable landing sites between South Africa and Perth. Thanks.

    • Are there scheduled direct flights between Johannesburg and Perth currently? ETOPS depends on the specific plane model but I can’t find anything scheduled non-stop. All the itineraries I can find quickly go north around the Indian Ocean with at least one stop in Asia somewhere, which makes ETOPS essentially irrelevant.

      Beyond that, I haven’t done the math, but I suspect you could route a plane within ETOPS range of Diego Garcia, esp. with the crazy long certifications they’re allowing these days.

      • I suspect a number of routes that existed 2 years ago have been canceled due to COVID — but I have a vivid memory of Qantas’s inflight magazine in 2010 showing nonstops from western Australia to southern Africa and from eastern Australia to the west coast of South America. I remember noticing this because my 1985 flight from LAX to AU had 1 stop going over and 2 going back — and I met someone who’d needed half a dozen legs to get from UK to AU. For a while the world was getting smaller.
        I don’t know whether those routes were served, then or later, by twinjets; all of my flights were on B747’s except for the one on an A380. It’s possible that the southern-ocean routes were flown only by quadjets.

      • Update: today’s news ( nudged me to check on Mauritius. The country is not as close east-west to the midpoint of Johannesburg-Perth as Diego Garcia, but it’s ~900 miles farther south, so it’s closer to a single-bearing course (let alone the shorter great-circle course) between those points. The airport has an 11,000-foot runway, so a plane in trouble could take off safely after diverting there.

        Zooming way out on Googlemaps shows scattered islands much closer to the midpoint of that route, but Wikipedia says they have minimal air facilities: see

        It’s not clear how many alternates would be needed, as Wikipedia notes there are ETOPS/EDTO certifications up to 5.5 hours:

        • CHip’s research inspired me to finally do some more digging – if I got the right flight, South African Airways in 2020 flew JNB to PER direct using Airbus 330-200s, which has ETOPS of 240, or about 1800 nautical miles / 3300km (if I’m doing my back of envelope math right.)

          It’s about 6000km distance between Port Louis on Mauritius and Perth, so that’s probably the diversionary airport for the flight once you’re out over the ocean – if you lose an engine, you either turn back to Port Louis, or you press on to Perth. No other airport is required.

  • Sylvia published a few useful comments, but I do not fully agree.
    The procedures that apply to an aircraft, in order to bring it from a cruising level or height to the final approach segment are laid down in the so-called Standard Arrival Procedures (STAR). ATC will take over at a certain point in order to guide the aircraft to the initial approach point from where it will line up with the runway-in-use for approach. At least, this is the usual way for larger airports with published approaches and radar services. Modern facilities and technology make it possible for ATC to give radar guidance for nearby smaller airports, even if the destination itself does not have local approach radar. The STAR will incorporate all minimum heights. As long as the crew adheres to the STAR and the following approach procedures, there will not be a conflict with terrain.
    When the aircraft is under VMC the crew will be responsible for this clearance, but what I understand by “let-down procedure” is a set of instructions, issued by ATC to the crew in order to safely descend to a certain platform bypassing published (STAR) approach tracks. The most important factor here is “guidance”: ATC will to a very large extent assume responsibility for the safety of the aircraft in relation to terrain avoidance and, of course, separation from other traffic. In other words: If I were to accept “let-down” from ATC I would assume that they in turn assume responsibility for my safety.
    And, like in all other situations, the crew must adhere to the instructions, or declare an emergency in order for the safety to be guaranteed.
    Of course, the element of “human error” is something different, if mistakes are made then all bets are off.
    This is my own interpretation. It need not be true. I am curious to what else, what other comments, will come up.

    • Hmm – It gets complicated. Read this it might help.

      Or maybe not!! It all depends on the type of service being received.

      However, as a general point – Responsibility for the safety of the aircraft ultimately sits with the Captain. No one else – and never ATC. In controlled airspace you MUST do what ATC say (outside it’s advisory) – but if you disagree on safety grounds it’s the Captain’s decision to do what he/she believes is necessary to maintain safety (and then justify it at the subsequent Board of Enquiry)!!

      See my general comments earlier on about the “procedure” which apply to aviation in general and not just aircraft approaching airports.

  • Frank, yes you are correct. But, on the other hand: in IMC and when under radar control, ATC is responsible for ensuring that an aircraft will remain clear of terrain, obstacles and other traffic that they are aware of.
    The captain has the ultimate authority, meaning that he or she can deviate from ATC instructions if there are grounds to do so. If (s)he has not given a sound reason, e.g. by declaring an emergency, there will no doubt be an enquiry. Especially if this action resulted in an incident. Sometimes it can result in a terse message from ATC: “Call me immediately after landing”. The pilot and the controller can then discuss it over a landline without the need to occupy ATC radio channels, and often the situation can be resolved. And if not, then indeed it may result in an enquiry.

    “Talking down” (not “Take-down”) can also mean an approach by radar. SRA: Surveillance radar approach is one. The pilot will be handed over to the radar controller who will take over from approach control. It is usually preceded by the controller identifying him- or herself as follows, as verbatim as I can remember:
    “This is radar approach control. Check your minima.” The pilot will state them. The controller will take over again: “Do not acknowledge my instructions. You are clear to land from this approach. Check your gear is down and locked. Start you descent now.” This will be followed by a series of heading corrections and altitudes to check that the aircraft is at the correct altitude at that moment. When the aircraft is at the point where the pilot should have visual contact, given the minima stated, the controller will remind the pilot that he or she has reached the minima and usually sign off and hand the flight to ground control.
    PAR or precision approach radar. This goes more or less the same way, but the controller will now also be able to monitor the glide path and therefore will be able to provide heading, as well as glide path deviation information. Again, the pilot will not respond during this phase but make heading and descent rate corrections as needed.
    A PAR approach is a precision approach and a flight can be guided accurately right down to the ground, The controller will say something like “start your flare now” and sign off as the aircraft has touched down. A PAR approach to zero limits is not usually approved for civilian pilots. They will still have to state their limits before establishing on the final descent path
    In the early days a (military) crew would be trained to land in zero visibility conditions. A transparent blue film would cover the windscreen, the trainee would wear yellow goggles.
    The instructor would still be able to see through the blue film, the student would have no problem seeing the instrument panel through the yellow goggles, But the combination of blue and yellow would make it totally impossible to see anything outside the cockpit. A very effective method and much better than the “hood” limiting the field of vision. But of course, the old method was also more costly. The hood can be used and re-used hundreds of times. The gelcoat filter over the windscreen has to be removed and discarded after training sessions. Besides, nowadays we practice Cat 2 and Cat 3 approaches in the simulator and, once qualified, have to log them in order to stay current.
    My own instructor simply brought old newspapers and tape to block off my side of the windscreen. It worked too, we went right down to “zero” during IFR training.
    Okay, that was about “Talk-down”.

  • I´d like to ask about ground radar, because there´s a lot of contradictory information online. Was it developed in the 1940s? How has it changed since then, and when were the transponders introduced? Where was the first airport to install it? (I have Orly in 1964 but there may be earlier ones.) Aren´t there two sorts, and what is the usual range of each; I mean from how far away could ATC at an airport locate a specific plane?

    • Elisa, ‘ground radar’ was used for a very short range radar that rotated very fast and was used by the ground controller at a major airport to verify where taxiing aircraft were in poor visibility; however, I’m not sure if the aircraft were actually controlled using the radar.

      I believe I’m right in saying that radar was developed during WW11, but was not the rotating radar we know today. These radars sent a pulse of power out from the radar head, the radar head then switched to be a receiver and waited a short time to see if the pulse was returned after hitting an object, this procedure carried on continuously whilst the head was being rotated. A timeline was shown on a CRT (Cathode Ray Tube) and if a target was detected, a bright light, or blip, came up on the timeline, the distance from the centre of the CRT was half the time between transmission and receipt of the radar pulse. This was commonly called raw radar. The distance the radar could detect targets was governed by the speed of rotation of the head, i.e. how long you waited for the blip to get back. Airfield ATC generally control within a limited radius of the airfield, maybe 40 miles, therefore, the radar can rotate quite quickly (say15 rpm); however, long range radar for air defence say, would rotate very slowly (say 4 rpm), but could see over 200 miles radius.

      Raw radar progressed to computerised processed radar, but still with a rotating radar head, nowadays I understand that it is all phased array , but that is way beyond my understanding.

      My experience was with the RAF in the 1970/80s.

      • Thank you so much. That is very helpful “Cathode Ray Tube” is certainly familiar. My father was an electrician, and he rented out televisions back in the days when they were all black and white and had valves. They also had Cathode Ray Tubes; I remember the boxes. This is a great page, isn´t it? I´m not a pilot, just a writer who loves planes, and I´m learning a lot here.

  • This was a wonderful idea of Sylvia. She unleashed a very entertaining number of reactions, which for me underlined how things that I (we) worked with routinely as part of our daily life can be much more complex than we thought. For one, I never gave “let down procedure” much thought. And although I was aware that as the captain I would be ultimately responsible for the operation of the aircraft that I was in charge of, it was more or les assumed that in controlled airspace ATC would bear responsibility for keeping me clear of traffic and terrain. I was only dimply aware that the “fine print” of the law implicitly gives ATC the tools to wriggle out of it in most cases if there is an incident or accident.
    Elisa’s question is another example. We are so used to radar that we give it, the history of its invention and introduction, little or no thought.
    The idea must have been formed in the ‘thirties. It also required a lot of research to put it into practice.
    Elisa mentions “ground radar”. She probably does not refer to the radar installed at major airports to guide pilots along taxiways at night, or during periods of reduced visibility.
    London Heathrow had a marvelous system that I have not encountered anywhere else. The controller simply instructed to “follow the greens”. Green taxiway centreline lights would guide the crew from block to block: if the pilot had to halt, e.g. to give way to crossing aircraft, the green lights at the next block would simply disappear and a red bar would illuminate across the path of the aircraft that had to give way. There was no need to talk, this system would bring the aircraft right on to the stand where the aircraft would be handed to the marshaller and the ground controller would sign off.
    But in its inception, Radio Detection And Ranging was not nearly as sophisticated. The aerials were fixed wires, later replaced by parabolic dishes. A lot of wires were needed in order to get a directional fix on the aircraft. WW2 of course was the impetus for a rapid development, the enemy attacked using the cover of night. Soon the operators were able not only to guide the RAF interceptors to the target, but also were able to establish the height using the angle of the beam. Having a distance and an angle allowed the radar operators, using simple geometry, to determine the height of the aircraft in thousands of feet. This was given to the interceptors as “angels”. How appropriate with Christmas only a few days away!
    At first, the ground equipment was only able to establish the distance of a (metal) object by measuring the time between sending the signal (in the UHF frequency band) and receiving the “echo” of the interrogating signal.
    The next step was the installation of a device that would receive the incoming radar beam and returning it in a modified form. This would show on the radar scope. The system was known as IFF (identify friend or foe). The modification of signal could be changed, so that the operators had a good picture that enabled them to keep control over the situation.
    After the war, when civil aviation started again, the advantages became apparent and IFF became known as “transponder”. It can now incorporate a lot of information about the aircraft such as call sign, altitude, track. The systems are increasingly sophisticated.
    And of course, airborne radar is now universal. Mainly used to locate and identify severe weather, it can also identify ground features. But with the number of other systems, including maps with lots of information projected on the pilots’ instrument panel, airborne radar is mainly used for detection and avoidance of severe weather such as thunderstorms. Pointing the aerial down to see ground features results in a lot of clutter that may mask nasty thunderstorms. We normally kept the aerial so that it would point slightly up, with the bottom of the beam just below the horizon (of the cruising level). We would scan lower airspace when descending.
    Maybe some of the readers have some more explicit data about the history and early days of radar?

    • Thank you Rudy. That´s very helpful. I had no idea it was so varied. Everything gets more complicated the more you find out about it, doesn´t it? Best wishes

      • I’m not a pilot, but I’ve had a long career in engineering. Over my time I’ve come to realise that every minute detail of anything your eye might catch probably was a huge frustration for an engineer, or maybe even exists only from the benefit of knowledge developed through multiple PhDs.
        Reminds me of this little comic:

        • Good question, and I´ll interested to see the answers you get. As you refer to FAA, do you mean just in the United States? It would be interesting to see a comparison worldwide.

          • Please ignore the comment above. It was a reply to something else. Apologies for any inconvenience; I´ve no idea how it ended up here.

    • Wow, that Heathrow system sounds amazing! This is the first I’ve ever heard of it, and it sounds like the thing to stop runway incursions.

      I wonder how it works from the controller’s POV? How expensive it was? And how much it “messes up”? (Yes, I’m a software developer and I know stuff breaks. Often)

  • wrt “ground radar”:

    Science fiction author Arthur C. Clarke worked on the development of ground-controlled approach during World War II; he later used the experience for his only non-SF novel, Glide Path. GCA is what I remember reading about (as a still-active system) in a ~1960 book on aviation; SRA, as described by Rudy, sounds like its first successor.

    I remember a surface-surveillance radar (60-100 revolutions per minute, coverage from the ground to maybe 50 feet up) being considered routine when I visited a major tower in 1975 (part of our instrument course), but I don’t know how far back it goes.

  • Do airliners ever fly over the north pole? If flying over the pole was the shortest route, would an airliner use it? Are GPS and inertial navigation the only worthwhile ways to navigate at very high latitudes?

    Some fighter plans have a thrust to weight ratio that is greater than 1 to 1. If a fighter plane flew straight up at max power, what would eventually end its climb, and what altitude would it reach?

    I would like to take a flying lesson in a C-130. What would it cost per hour? How much for an hour of instruction in a DC-3?

    If a parachutist opened his parachute at 15000 feet, would he take an hour to reach the ground? Do parachutists ever do this?

    Why doesn’t the engine torque make a single engine plane roll uncontrollably?

    • If you post these six questions as six individual comments, they will generate six useful threads of answers. As it now stands, the answers will all be mixed into the same thread, greatly reducing their readability and usefulness.

    • The lift generated by the wings counteracts the engine torque on fixed-wing aircraft. For rotorcraft, there’s a second rotor, usually the tail rotor, with this job.

      As a jet gains altitude, the air around it gets less and less dense. I expect that the thrust of the engines would decrease due to lack of oxygen to burn, and the jet would become hard to handle as the control surfaces become less effective.
      The altitude record stands at 37,650 metres (123,520 ft) and was set by Aleksandr Vasilyevich Fedotov, in a Mikoyan Gurevich E-266M (MiG-25M), on August 31, 1977.

    • I did a little skydiving ~30 years ago; never qualified even for the basic US license, but followed the news about the field for some time afterward. My impression is that in general skydivers don’t open at that kind of height; the obvious reason is that having to make quick decisions (e.g., how to untangle a line twist) when the oxygen level is low would be hazardous. (Yes, some people have climbed much higher than that without supplemental oxygen. They train, and they never leave the ground, so they’re more capable of dealing with low oxygen and have more time to make decisions. Even the record jumps with oxygen, from 100,000 feet or more, tended to open well below 10,000 feet.) People have done large (>200) “free-fall” formations starting from ~18,000 feet, but they were on oxygen until they left the airplane — and most of the formation left other planes lower down as the high exiters were making the core of the formation.
      Wikipedia tells me that the military does do very-high-opening jumps, as a way of inserting single people where a plane could be seen and shot down; see for details.
      Another exception would be Canopy Relative Work, in which people with fully-open canopies attempt to make formations (generally vertical, rather than horizontal as in “free-fall”.) mentions a group exiting at 14,000 feet; my guess is that they would have opened quickly to get more time to form up. The picture at that link is at a bad angle; for a dramatic view of a 100-piece formation, see

      ISTR that a modern (“square”) canopy descends at ~1000 feet/minute near the ground, so someone who opened at 15,000 feet would be up less than 15 minutes; I’d expect them to descend faster at first because the air density is ~50% of sea-level density, but I don’t have the math or the tables to be more precise.

    • In the fifties over land, there were radio beacon aids to navigation (NDBs, VORs). Once on the ocean or over remote areas, navigation was by use of Loran, Consol (both using long range radio signals) together with astro and, all too often ‘dead reckoning’ (plotting a track based on the forecast winds). In daylight, over remote land areas it was not even unusual to resort to map reading.

      Most fixing methods required plotting position lines on a chart which of course took time. Especially at night, astro navigation came into its own; but a ‘three star fix’ involved 10 plus minutes of calculations, followed by three sights using a bubble sextant, each lasting exactly two minutes and separated by two minutes exactly (the sextant was deployed through a port in the flight deck roof, which replaced the ‘astrodome’ on previous types). The result of all this effort was a three position line fix – but by the time it was plotted on the chart the position was valid fifteen minutes ago.

      Even Loran, Consol and ‘dead reckoning’ would not produce an instantaneous fix.  In the later sixties the 707s and VC10s of BOAC were fitted with Doppler which at least gave accurate tracking and groundspeed information. It was not until the Boeing 747 entered service in 1970, with inertial navigation built into its management systems, that a crew could know their instantaneous position.  

      Ian Frow,

  • My question relates to the replacement of three person crews in commercial heavies with two person crews.

    In the 1960s, 70s, and 80s I believe that western commercial heavy/widebody airliners all flew with three person flight crews. Smaller commercial airliners, under 80,000 MTOW?,were permitted by FAA to have two person crews, but I don’t have a specific reference for that.

    Did the relevant agencies study smaller crew sizes, presumably due to better avionics in the more modern planes, in any rigorous way before these regulatory changes took place? What is the history behind this change?

    • Good question, and I´ll interested to see the answers you get. As you refer to FAA, do you mean just in the United States? It would be interesting to see a comparison worldwide.

      (I wrote this comment before, but it ended up in a different place altogether.)

      • I was referring specifically to the US FAA because I found a mention of 2 person crews allowed by FAA in early narrow body certification, which is where I saw the 80,000 MTOW mentioned as the cutoff. A brief web search did not turn up primary sources however.

        I don’t know what the individual European agencies, prior to the formation of EASA, did for these regulations.

      • I removed it but yes, the replies and commenting on here aren’t great for serious discussion. Still, I’m loving how we are managing to make it work!

        • Thank you (I did try but was worried about erasing things. It has happened before.). This discussion place is doing well, and I hope it keeps going. Lots of interesting information.

    • From the mid-fifties to the present, the B-52 Bomber, 488,000 pounds max takeoff gross weight with eight engines and around 240,000 pounds of fuel, was flown by two pilots…no flight engineer, flight mechanic, etc.

      • What other minimum crew besides 2 pilots, if any, was needed in early models B-52B through B52-F?

        Obviously a mission crew was larger, including bombardier and gunners. The defensive machine gun armament evolved over models and were eventually replaced with remote control cannon and some gunner crew positions were eliminated or changed.

        • The other minimum crew member was one of the two navigator bombardiers. This was mostly to have an open hatch available for a manual bailout in the event of ejection seat failure. Both of the navigators had downward ejection seats leaving an open hatch for a manual bailout.

          For normal takeoffs, the navigator ran a stopwatch from 70 knots for a predetermined time to a predetermined speed to verify that the aircraft was accelerating sufficiently. It was normally around 10 seconds to about 110 knots. So the navigator would call out “9.8 seconds NOW” and the pilots would check that they had more than the required airspeed. Also, the navigator would crosscheck the Best Flare Speed (based on gross weight) with the co-pilot for landing.

  • Wow, that sounds like an “exciting“ way to takeoff. Especially that the first few series had relatively underpowered turbojets with around 10,000 lbf each.

    Was protocol for a full conventional mission load? SAC nuclear missions would have had a different equipment profile.

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