Ask the Captain: What happens when all systems fail during flight?

— -- Question: Captain Cox, passenger jets have installed safety / back up systems to prevent a partial or complete systems failure while in flight . Has a jet ever experience a complete systems failure while in flight ? When all emergency back up systems have failed either one by one , or completely . And is it possible to recover from a complete systems failure while in flight ?

— submitted by Mark Johnson, Livingston , Texas

Answer: There have been a very few cases of multiple system failures. One example is United 232, the DC-10 that lost all hydraulic systems due to an uncontained engine failure; another is QANTAS 32, the A380 with multiple system failures due to an uncontained engine failure. The DC-10 was unable to recover hydraulic power but was able to find an alternative method of limited control. The QANTAS A380 did not lose everything but successfully landed with many systems compromised and only limited control.

The traditional standard is that triple redundancy is sufficient. On critical systems, the engineers have calculated that there is less than a one in a billion chance of simultaneous failure. That is the certification standard.

The answer to your last question of whether it's possible to recover is "sometimes." It depends on the system failures and if they can be rebooted, restarted, or powered by an alternate means. An example was the British Airways B747 that flew into a volcanic ash cloud at night. All four engines failed. After descending out of the ash cloud the crew was able to restart the engines and land successfully. While the Air Transat flight that had a massive fuel leak and had both engines quit near the Azores was unable to repower the airplane, requiring a no-engine landing. Two different examples of the ability of a flight crew to recover a critical system, engines, but both with successful outcomes.

Question: (note* English is NOT my primary language, I am sorry for any grammar errors )

1. Since a jet engine sucks in air, it will obviously be able to suck small items in the engine. What is a safe distance to be (from it) when the engine is running?

2. Why are the cabin lights off during takeoff and landing?

3. Can the pressure in the cabin of an aircraft cause damage to your hearing? If so, what can you do to prevent this from happening?

4. The Paris Concorde crash was caused by debris on the runway, how often are runways checked for debris?

5. Is the stall speed of an aircraft determined on the maximum takeoff weight?

— jptiger

I will try an answer your questions:

1. Jet engines intake a substantial amount of air when they are operating. There are warnings posted on the nacelles of engines that show the safe and unsafe areas around it. Jets do pick up objects (rocks, foreign objects, debris, etc) during normal operation. It can cause damage to the fan blades requiring repair or replacement. The safe distance of a jet engine operating at full power varies from engine to engine but it would be a large distance.

2. Cabin lights are turned off to reduce the amount of light difference between the exterior and interior if an evacuation is necessary. The lower interior lights allow for your eyes to more rapidly adjust if you have to evacuate during night operations.

3. I do not know of any evidence that shows damage to hearing from aircraft pressure.

4. Based on airport operators I have talked to, runways are checked at least daily and usually multiple times a day for debris.

5. A stall occurs when the wing exceeds the critical angle of attack. This causes a rapid loss of lift, and airflow distortion across the wing. The speed at which critical angle of attack is reached will vary with weight, wing configuration, and the G load on the airplane. Certainly an aircraft at maximum takeoff weight has a higher stalling speed than a lighter weight one, but exceeding critical angle of attack is the cause of the stall.

John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.