1. For many commercial aircraft, there is an emergency procedure for when pitot static based flight instruments become inoperable.
Pitot static based flight instruments include airspeed indications, altimeters and rate of climb instruments.
2. The scenarios encountered that might render these systems inoperative include a blocked pitot tube or a blocked static port.
A. In the past safety investigations found the following factors involved in pitot tube blockage:
1. loss of pitot tube heater allowing ice to build up or precipitation to clog the tube and/ or the associated plumbing.
2. a protective maintenance cover left on the pitot tube
3. insect, bird or other debris or object entering the pitot tube.
B. In the past, safety investigators have found the following factor most commonly involved in static port blockage:
1. Masking tape place over the static port by crews washing, waxing or painting the plane, where the tape was not removed prior to flight.
3. Since from what has been reported in the media that the flight was proceeding normally, it can be deduced that the problem with the pitot and static system was most likely due to the pitot tube icing over due to lack of heat.
A. Since most pitot heat systems are electrically powered, it is possible that there was some interruption in that electrical system.
B. When the loss of pitot static powered instrumentation occurs, the flight crew is directed by emergency procedures to use instruments which indicate flight attitude, that is pitch, roll and yaw.
C. The attitude instrument most often found in jet powered tranport aircraft is the attitude indicator. It will simultaneously indicate pitch, roll and yaw.
D. Attitude instruments are most often powered on commercial jet transport aircraft electrically and therefore will provide valid data in the event the pitot static system is inoperative. Their source is either laser ringed gyros, mechanical gyros or other similar systems.
E. Laws of aerodynamic performance state that pitch-attitude controls airspeed and engine power controls altitude. So as long as the flight crew maintains the cruise pitch attitude and the cruise power settings on the engine, the aircraft should stay relatively level in flight and the airspeed should remain at the speed required for cruise flight. The crew is often directed to seek an area of clear sky outside of icing conditions or precipitation in an attempt to regain use of the pitot-static system in the event icing caused the problem.
4. This is an emergency procedure which is successful and will allow the flight crew to maintain control of the aircraft through all flight regimes. I can speak from experience that this procedure works just fine. I can also state that this procedure is practiced in training simulators at many US airlines and I would suspect at many European airlines as well.
5. If the investigators state that the aircraft stalled, there are many scenarios by which this could have taken place. One common scenario is that the pitot tube ices up decreasing the dynamic pressure input to the airspeed indicator and rate of climb indicator. The static pressure port may not be blocked so it continues to show static pressure. If the crew does not cross check the pitch-attitude indicator, and only looks at pitot-static instruments, they may see an increasing airspeed and react by increasing pitch and reducing power. This could lead to as stall within a short time at altitude with an aircraft heavily laden with fuel, passengers and cargo.
6. To prevent this type of mishap, many airlines employ training in emergency procedures for the loss of pitot static instruments. The procedure includes disconnecting the auto pilot from control of the aircraft and hand flying the aircraft, again using pitch attitude and engine power settings from a chart. The charts carried on the aircraft include variables such as flight altitude and aircraft weight.
7. If the loss of pitot static system occurred while in a severe thunderstorm, the crew would have had to deal with both the severe turbulence, icing, possible lightning as well as the disconnecting of the autopilot. That would have been a handful for any crew to handle, but that is why most major airlines have strong training programs.