Aviation Airspeed Guide
Just a quick one today but I really liked this guide to the airspeed indicator which was published to r/aviation.
The thread has ended up in an interesting discussion about Vne (the never-exceed speed) and why the gauge goes up to 200 on an aircraft which can’t (or shouldn’t) exceed 155 knots. The comments range from the helpful to outright silly.
The airplane is not structurally rated for 155+, the aerodynamic stresses are dangerously high for that aircraft at that speed.
Are you legally obligated, once you reach high yellow zone, to announce, “She can’t take much more, captain!”?
The diagram in the link below breaks the limitations of an aircraft down into a diagram that gives a better visualization of the green and yellow arcs on an airspeed indicator and the red Vne line. It takes a bit to compare an airspeed indicator to the diagram, but this was been one of the best learning tools that I have seen to compare structural limits to the airspeed indicator.
Some pilots prefer to use the whole speed indicator. The wings aren’t nearly as courageous however, and decide to watch the whole thing from a short distance behind the plane.
I read this in David Attenborough’s voice. I could listen to this for hours.
Oh god, it would be amazing and tragic at the same time.
And here we have the majestic cessna, taken its final landing as it plunged itself into the banks of the river Thames. Small vessels like this one are prone to being taken by the turbulent winds of England. One minute it was soaring freely and the next it is surrounded by the bustle of a pack of crash investigators. Bystanders look on in horror as the wreckage is pulled apart piece by piece, revealing the mangled and malformed structure that once carried life through the skies. This is all a part of the cycle, of the living, breathing, planet earth.
On jets and large turboprops there are no painted regions or other markings. This is mostly due to the massive variation in what the limit speeds can be based on combinations of weight, altitude, air temp/pressure, etc.
Every airliner I’ve flown has what’s called a barber pole for the max speed (it’s red and white striped needle, like what you’d see outside a barber shop). The barber pole shows the never-exceed speed for the current conditions. So it will move depending on atmospheric conditions and altitude, and changes from a max indicated speed to max Mach speed at a certain altitude.
There is no caution range in jets/turboprops either. Not sure why, you’d have to ask an aeronautical engineer. Stall speed will vary so much on big planes that it’s not permanently marked on the airspeed indicator either. On the 767, max takeoff weight can be 440,000lbs, and lowest with just 2pilots and minimal fuel is 220,000lbs. Minimum speed with flaps up in those examples can be as high as 265kts, or low as 195kts.
Old school gauges have bugs we move to show minimum speeds for each flap setting, and digital gauges show the flap bugs along with a low-speed awareness queue. That can look different based on whose avionics are in the plane. The E-170 has a white/amber/red line in the speed tape showing how close to or into the stall you are, but on the 757 it’s just a little amber bracket above the bottom barber pole.
Flap speeds aren’t posted on the gauge either, not in any permanent way. However, it is something we have to know. I fly 4 variants of the 757/767, and each variant has different flap limit speeds. Our airplanes also have both an old-school airspeed gauge and an electronic speed tape, and on the speed tape the max speed for the current flap setting is shown by the barber pole moving down to that speed. For example, Flaps 20 on the 757-200 is 195kts. Once I’ve selected that, the barber pole on the speed tape moves down to 195 from where it was previously. The barber pole on the OG airspeed indicator does not, however.
Hope that helps a little
I had just gotten out of the Air Force when a friend asked me to deliver a Citabria for him. I’d never flown a tail dragger so he arranged for an instructor to take a few trips around the flag pole with me to show me the technique.
The instructor was about eighteen and somewhat humorless. He decided to give me a little quiz right after takeoff, and so I heard over the interphone from the back seat, “OK, can you tell me the stall speed of this aircraft?”
(Now, you have to understand the one-g stall speed is pretty meaningless to the military…they emphasize that the aircraft stalls at any speed when you exceed the stall angle of attack. I had completely forgotten about the white arc on GA airspeed indicators.)
It seemed like a strange request at 300 feet on takeoff leg, but I said, “Sure”.
I pulled off some power, pulled the nose up until the airspeed bled off, then recovered from the resulting stall. “It’s about 50”, I replied. “Want to see it spin?”
He didn’t ask me any more questions.
A bit of light-hearted fun but also it was intriguing, to me, to see the amount of general interest in how planes work.
I do have one question, though, which is why the hell is the airspeed indicator in mph and not knots?