MH370 Search: Beacons and Pingers and Locators

28 Mar 14 7 Comments

The press is continuing to speculate as to causes and criminals, but I think we’re all clear now that until we find the aircraft itself, we won’t know what happened. Of course what everyone is hoping for is that we find the “black box” and that it has useful information on it.

A black box is actually a bright-orange container designed for high visibility, which houses the cockpit voice recorder and the flight data recorder. The black box is housed at the rear of the aircraft, on the presumption that following the initial impact, the rear of the aircraft will be moving at a slower speed. The black box is engineered to survive a catastrophe, including crashing down to the bottom of the South Indian Ocean. It is extremely likely that if we find the black box, the contents will be safe and we will get at least some data on the final flight of Malaysia Airlines Flight 370.

However, finding the black box is proving difficult because we simply don’t know where the aircrash was.

Aircraft are fitted with distress radio beacons, often referred to in aviation as ELTs (Emergency Locator Transmitter) or more colloquially as pingers. These beacons send out a distress signal every second in order to help search and rescue determine the location of a downed aircraft. Traditionally, a distress beacon would interface with the International Cospas-Sarsat Programme, a search and rescue satellite system established in 1979 by Canada, France, the United States and the former Soviet Union. However, it isn’t possible for a distress radio beacon to broadcast to a satellite from underwater. The point of the distress beacon was to find survivors as quickly as possible. It was not intended to discover sunken wreckage at the bottom of the ocean.

In 1961, the UK Ministry of Aviation focused on how to locate and recovery aircraft lost in deep water, with the result that commercial aircraft since 1988 carry mounted acoustic beacons for underwater use. All modern commercial jets now carry an Underwater Locator Beacon (ULB). In the photograph above, the Underwater Locator Beacon is the small cylinder on the far right.

A ULB is powered by a lithium-ion battery. Once the beacon is immersed in water, the water closes an electric circuit and the beacon begins to transmit. The ULB will transmit a “ping” at an acoustic frequency of 37.5kHz every second at full power for 30 days. The detection range is 1-2 kilometres in normal conditions and 4-5 kilometers in good conditions. After the 30 days, the ULB will continue to transmit but the range will reduce day by day until it stops altogether. How long it will continue to transmit is based on various factors, including the environment it is in and the age of the beacon and the battery itself which is generally replaced every few years.

After the Air France Flight 447, the Bureau d’Enquêtes et d’Analyses (the French Bureau of Enquiry and Analysis for Civil Aviation Safety) recommended that the ULBs’ transmission period be increased to 90 days.

Honeywell Aerospace, the producers of the black box on Malaysian Airlines Flight 370, have confirmed that the cockpit voice recorder will only have the final two hours of the flight on it. However, the flight data recorder will allow us to recreate the flight itself and the wreckage itself may help to unravel the mystery. The ULB will be attached to the black box and we’re hoping it will lead us to the wreckage of the aircraft in the South Indian Ocean.

Hydro International describes deep-water black box retrieval as “A game of hunt-the-pinger against the clock.”

Deep-water Black Box Retrieval – November 2009, Volume 13, Number 09 – Archive – Hydro International

Localising a pinger from the surface in shallow water is relatively easy, as described above. This task becomes increasingly difficult as water depth increases, however, because the direction is affected by both the horizontal bearing and the depression angle to the beacon (Figure 2). When trying to locate a pinger beacon in deep water, the detection equipment should be installed on a self-propelled underwater vehicle (either an ROV/AUV or a manned submersible). However, this presupposes that the position is already known to within the maximum 2-3km detection range. When aircraft debris is scattered over a large area, as with the recent Air France 447 accident off the Brazilian coast in depths up to 3.5km, a grid search must be conducted using underwater acoustic listening equipment. This equipment must be deployed as deep as possible to overcome the bearing/depression angle conflict (such as on the nuclear submarine described in a news feature in the July 2009 issue of Hydro International). The additional time required to mobilise and carry out this search highlights the second major limitation of fitting CAT aircraft with pinger beacons: that of their limited operational life.

Last week, two different search mechanisms were moved into place in the South Indian Ocean: a Towed Ping Locator 25 and a Bluefin-21, an Autonomous Underwater Vehicle.

The Towed Pinger Locator 25 will be operated by a team on the Royal Australian Navy supply ship Seahorse Standard. The ray-shaped sensor searches for emergency relocation pingers on downed aircraft up to a maximum depth of 20,000 feet.

The US Navy Fact File: Towed Pinger Locator 25

The system consists of the tow fish, tow cable, winch, hydraulic power unit, generator, and topside control console, although not all of these components are required on every mission. Navigation is accomplished by using algorithms incorporating the amount of cable in the water, the depth indication from the pressure sensor and other parameters. The generator provides electrical power for the system or power from the support platform can be used if it is compatible. The tow fish carries a passive listening device for detecting pingers that automatically transmit an acoustic pulse.

The Pinger Locator is towed behind a vessel at slow speeds, generally from 1 – 5 knots depending on the depth. The received acoustic signal of the pinger is transmitted up the cable and is presented audibly, and can be output to either a Oscilloscope, or Signal Processing Computer. The operator monitors the greatest signal strength and records the navigation coordinates. This procedure is repeated on multiple track lines until the final position is triangulated.

The Bluefin-21 automous underwater vehicle is a sonar-equipped robot used to search for transmissions from the Underwater Locator Beacon as well as detect debris on the ocean floor in an attempt to find the wreckage of MH370. The torpedo-shaped vehicle can operate almost up to three miles underneath sea-level and uses an acoustic camera to provide very high resolution sonar still imagery and video.

Diameter 53 cm / 21 in
Length 493 cm / 16.2 ft
Weight (Dry) 750 kg / 1,650 lb
Depth Rating 4,500 m / 14,763 ft
Endurance 25 hours at 3 knots
Communications RF, Iridium and acoustic;
Ethernet via shore power cable
Data Management 4 GB flash drive for vehicle data
Plus additional payload storage

Last week, I was a guest on a radio show where I explained that trying to find the aircraft wreckage underwater with these locators was looking for a needle in a haystack. “It’s worse than that,” interrupted an ex-NTSB investigator. “We don’t even know where the haystack is yet.” That’s a pretty perfect summary of the situation.

The limited range and speed of the Towed Pinger Locator 25 and the Bluefin-21 mean that they are of little use in a large area. A vessel towing a pinger can search about 15 square nautical miles per hour in depths less than 2 km. As the water gets deeper, the grid-search becomes slower.

The current search area, moved today to a zone about 1,850 km west of Perth, is approximately 319,000 km². Even with two ships searching to a depth of less than 2 km, we’d be talking about over a year of non-stop, uninterrupted searching in perfect weather. Unfortunately, the South Indian Ocean is quite a bit deeper with an average depth of 3.9 km, and the Underwater Locator Beacon will start getting weaker in ten days.

The reason that the TPL-25 and the Bluefin-21 are in place is so that if we do find debris, they are ready for action rather than losing more precious time transporting them to the scene.

Right now, though, all our hopes are pinned on the ocean surface search for debris. The photographic imagery captured today is being assessed overnight and weather conditions for Saturday are expected to be “reasonable”.


Previous articles on MH370:

Unravelling the Theories Behind the Disappearance of MH370

Considering the Probabilities of the Fate of MH370

7 Comments

  • Given the amount of times we’ve heard “Look, debris! Wait, we’ve lost it” I doubt very much they’re even going to find the haystack.

  • Congratulations Sylvia. I can see why you are a journalist. A very good explanation, but as a (former) airline captain I see no reason why everyone persists in calling the recorders “black”.
    The press, if they need to sell newspapers will invent stories if they lack anything concrete to tell about.
    What I have been afraid of is that some day in a far-away village a tribesman consults the tribe’s elders. An aircraft has crashed in the vicinity.
    He has found a bright orange object and wants to use it for some purpose. The chieftain tells him not to worry, he owns a radio and heard that “they are looking for black boxes. This thing is orange so you can use it”.

    To-day I heard another news item that sounds as pure spoof:
    Investigators are now concentrating on an area much closer to the last known position of flight MH370.
    The press mentions that “the aircraft had been flying much faster than originally thought so it has used more fuel”.

    Folks, I have logged about 22000 flying hours.
    About 9500 were on turboprops, about 6000 on jets. From Citations and Learjets to BAC 1-11.
    The rest were on piston-engined aircraft.

    So I have hoisted my colours to the mast.
    Jet aircraft are most efficient at higher altitudes, above 30.000 feet. A typical cruising altitude would be 35000 – 37000 feet (or flight level 350 – 370).
    Once above 15.000 feet a typical jet engine will tend to increase it’s rpm. Either the crew, or computers will set and continuously adjust the power to allow the aircraft to continue it’s climb to the assigned cruising level without exceeding it’s design limits.
    Aircraft also have a minimum speed below which it will stall. And: they have a maximum speed. At lower altitudes this will be expressed as VMO or maximum operating speed (the “V” is a symbolic indication for speed). Somewhere above 20.000 feet, depending on the aerodynamic characgteristics of the wing, the MMO or maximum operating Mach number will be the determining factor. The Mach number is a ratio of the speed of sound which decreases with increasing altitude. A typical cruising speed for a jet airliner is M.82 or 82% of the speed of sound at a given flight level.
    In the rarer air at high altitude the stall speed increases. But the maximum speed decreases.
    So pilots will have to fly a fairly accurate speed if they want to safely and efficiently climb to a higher level because they may be close to the stall speed.
    The Mach number is limiting because the aerodynamics of a wing will change when approaching and exceeding the speed of sound.
    When this should happen, an aircraft may become uncontrollable.
    All this may sound very complicated – and to an extend it is.
    But the outcome of all this is: A jet aircraft tends to fly high because in the thinner air the drag reduces and with it the fuel consumption.
    The engines will at that point – worked out by computers and available to the crew as a read-out on the instrument panel and in older aircraft laid down in manuals – provide enough power to allow for an efficient cruising speed.
    There is not enough leeway for any significant speed increase nor to fly at a much lower speed.
    The cruising speed often is affected more by changes in the air temperature than by deliberate action of the crew. Speed changes (restricted between usually M.78 and M.84) are more likely to be the result of ATC for spacing.
    A significant reduction in speed will not really result in large fuel savings, unless flying in a strong tailwind.
    The scenario makes it unlikely that whoever took over command was much concerned with optimum wind and the flight path was not following a particular pressure pattern insofar as I could ascertain.
    So it looks to me, assuming that whoever was in control had some basic flying skills, that the crew were trying to gain altitude in order to maximise the range.
    This would have slowed the aircraft down but also could have eventually caused it to stall.
    Stall recovery is effected by a system called “stick shaker”, followed if no action is taken by a “stick pusher”. This will push the nose down in order to gain airspeed and allow the aircraft to recover.
    But at high altitude, perhaps too high for the aircraft’s actual weight, the aircraft would also have been close to the earlier mentioned MMO. Recovery may well have been too difficult for a hijacker to handle and may have caused a mid-air break-up of the aircraft.

  • Rudy, I definitely agree that there’s something very wrong with the press references to the aircraft flying faster.

    The thing is: We know that the aircraft kept flying until at least 08:11, so if it were flying faster but for the same amount of time, it’s clearly not going to have travelled less distance. The statement only makes sense if we don’t have time-travelled as a data point.

    Something seems to have been lost in the translation on that score, unless I’m missing something.

  • Yes Sylvia,

    There is something in the press reports that “jars”, a confusion that contradicts facts.
    But then, the discovery of a new debris field so far has again failed to yield any tangible results so far.

    Why is reducing speed in order to reduce fuel consumption futile in a jet aircraft?

    If we consider a car, this will work just fine.
    All vehicles are affected by all kinds of friction. Internal friction of engine and transmission, the wheels on the road surface and the air resistance. All cause drag.
    The drag from the air rises exponentially with increased speed, so it makes sense to drive more sedately if one wants to save petrol or diesel.

    It does not quite work the same way in a jet airplane.
    A car does not have to worry about being suspended in the air by a force called “lift”. Unless it falls in a large sinkhole, it’s wheels support it’s weight.

    An aircraft has to be kept aloft by lift, generated by air flowing over the wings.
    There are two factors: The wing profile, the shape of the wing and the angle of attack.
    The angle of attack, to a certain maximum, can be instrumental providing lift. But the higher the speed, the more the wing profile takes over and the angle of attack can be reduced.

    This, in spite of rising drag due to the speed increase, has the side effect of making the airplane more aerodynamically efficient and therefore the drag does not rise at the same rate as one could expect.

    But an aircraft has another trick up it’s sleeve:
    Up to a certain extend it has the ability to fly higher, in air of lesser density which in itself causes reduced drag whilst the speed, expressed as “true airspeed or TAS”, actually rises.

    The problem arises when a pilot tries to climb too high – see my previous contribution – where it can get in very rarified air where both minimum speed (stall) and maximum speed (MMO) are coming close together.

    My guess is that the aircraft was hijacked.
    How ? By whom ? Why ? We do not know.
    Did passengers try to interfere and storm the cockpit ? We do not know.
    Did passengers try to use their mobile phone in order to warn anyone ?

    Perhaps. A lot is made of the absence of mobile phone calls from the aircraft.
    But did they know the country access code of the mobile phone system that might have been in range ? Did they have a contract with a provider that honoured international calls between them and callers from a country like China, the Netherlands or other nationalities ? I am no expert, I do not know.

    My guess is that the aircraft slowed down in an attempt to gain altitude and thereby range.
    This would seem to explain why is slowed sown and other reports that put it at 45000 feet (FL 450). Very high for a 777.

    The altitude can be calculated from primary radar returns by simple triangulation: Distance from the aerial, angle of the aerial and the 3rd leg would be the (approximate) altitude.

    Did the hijackers at the controls overdo it, got too slow, decided to override the stick shaker and got into the stick pusher ?
    Did the aircraft subsequently enter an upset from which it could not recover an broke up ?

    Unless wreckage, preferably including the RFDR and CVR, is found: we do not know.

    The way this is going, we may never know.

  • Please forgive my ignorance, but would it be possible to adapt the black box so that should it be submerged in water a hydrostatic charge would release it from the aircraft, float to the surface and the last information being the exact location being when it was released?

  • Bear in mind that a key aspect of a black box is survivability. It has to withstand a crash of unknown intensity and direction. So extras are hard to add in, especially pieces that detach, I believe.

  • I still have a bit of an aversion against calling a bright orange box “black”, but anyway it’s transmissions that were supposed to aid speedy recovery now will have ceased.

    But although the chances of finding it, and the wreckage of MH370, have decreased dramatically, there still is a glimmer of hope.

    In a scientific magazine I read an article that suggests that there is one last technical avenue that has not yet been fully explored:
    Modern aircraft regularly transmit technical data to the maintenance base.
    These transmissions are totally automated and not under the immediate control of the cockpit crew.

    Apparently, these transmissions continued well after all other transmissions from the aircraft, including the transponder squawks, ceased. In all probability intentionally as a result of unlawful interference.

    Although these transmissions are non-directional, engineers have worked out a way to use them to get some information about the aircraft’s track from them.

    The first task was to establish who – what facility – did receive these signals and the hope is that they were stored in more than one airline maintenance department’s computer systems. It all depends on whether the airline concerned has the technical ability and software for the 777.

    With these data, the engineers can examine the doppler shift of the transmissions.

    You are all familiar with this, even if you do not know the term.
    Simply put: Suppose you are on a road and a motorcycle comes towards you. The louder and faster, the more convincing: as long as the vehicle is approaching you, the sound will have a somewhat high pitch. When passing and then moving fast away, there will be a very noticeable change in the note of the sound towards a lower pitch. This is the Doppler effect. It can be explained as follows: the sound waves coming towards you will appear to be compressed as the bike moves in your direction and will seem longer when moving away, which causes a distinct change in the sound.
    The same happens with radio waves.
    If transmissions are indeed received and stored from the onboard computer system, then not only will the engineers have a record of how the aircraft functioned, but also whether or not it moved towards or away from that particular receiver. If, as is hoped, more than one station received and stored the data, then triangulation can provide a rough last position of the aircraft.

    If this works, then at least the search area can be narrowed down and perhaps the wreckage located.

    Sadly, it is now beyond any reasonable doubt that the aircraft has crashed somewhere in the Indian Ocean with the loss of all on board.

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