Originally Posted by CT8
Astro would the speed reported be top speed at that altitude?
The speed reported by air traffic control was in excess of 300 knots, reaching about 350 knots as the airplane dove into the ocean.
There are a couple of speeds to consider:
1. Airplane maximum speed - I'm not a 737 pilot, but it's about 350 knots indicated* for most airliners.
2. Regulatory maximum speed - in the US, and many, but not all, countries, you're restricted to 250 knots indicated below 10,000 feet.
3. Best speed - in those countries where you're not restricted, the FMC will calculate a most efficient climb speed, which might be as much as 330 knots indicated.
When looking at Lion Air's profile, they were flying above the regulatory speed, which tells me there was a problem with the airplane. Particularly the instrumentation or the flight controls themselves.
Boeing just issued a bulletin for all 737 operators to treat an AOA** failure the same way that a pitot static failure is treated, and to disconnect the trim.
Here's why. In the 737, the stall protection uses the elevator feel system. That system provides the "neutral" position for the yoke. If the airplane senses an impending stall, then it will both trim the elevator nose down and increase the forward yoke using the elevator feel position. In other words, if the airplane senses a stall, it will push the nose down.
But, what if the stall indication is wrong? It's based on AOA, which is determined by probes on the nose of the airplane. Boeing's bulletin suggests that the probes on this airplane were faulty. The investigation to date has found that the airplane had a history of erroneous AOA indications.
One of those previous failures, in particular, tells you why I would never fly Lion Air - the crew had an AOA failure, struggled for control, and when it cleared up
continued on to their destination.
What they should have done was land the airplane immediately. You don't take an airplane that has problems, serious problems, on a long flight, unless you're an idiot, or pressured by management (which makes you a fool) or are just too poorly trained to understand the severity of an instrumentation failure.
So, AOA tells the airplane that there is a stall, and in the 737, airplane responds by pitching down, adding forward elevator feel, which holds the nose down. Full down elevator trim in a 757 takes about 75 pounds of force to overcome. So, when I say "feel", I am not talking about a gentle pull, it's serious force. In this scenario, which might be what caused this crash, the crew was struggling against an airplane that had many pounds of forward/nose down yoke pressure brought on by a faulty AOA reading.
As they analyze the black boxes and wreckage, we will know more.
But like Air France, the airplane gave the pilots bad information. In this case, the 737, that bad information leads to the airplane forcing the nose down, when it really shouldn't force the nose down.
I can see how the crew would have a hard time interpreting what is taking place, and given the magnitude of forces, struggling to control the airplane.
Time for me to go hit the gym...
*Indicated airspeed (abbreviated KIAS for knots or just IAS) is a measure of dynamic pressure; the difference between the pressure at the forward port of the pitot tube and the static port pressure. It's close to true airspeed when at sea level on a standard day. The airframe responds to dynamic pressure, so it's the most used measure of airspeed. At higher altitudes, Indicated airspeed is much lower than true airspeed. The air molecules are hitting the airplane faster, but there are fewer of them, so the pressure difference as measured is lower.
**AOA - Angle of Attack. The angle between the relative wind (the direction from which the air is hitting the airplane) and the mean chord line (a reference line based on the wing shape)... The wing performs exactly the same based on AOA, not necessarily airspeed. You could be going straight up in an F-14, at zero AOA, or at maximum AOA, it depends on what you're doing with the pitch controls and how much indicated airspeed you have to work with. You could have your nose on the horizon, and be deeply stalled (high AOA) or you could be flying at near zero AOA. Again, it depends on what you're doing with the pitch controls, and how much energy is on the airplane.
https://en.wikipedia.org/wiki/Angle_of_attack
