2
$\begingroup$

What kind of extreme weather would justify a pilot of a commercial airliner taking avoidance measures to the point of likely stall?

$\endgroup$
  • 6
    $\begingroup$ You are assuming that the events leading to the "point of likely stall" are deliberate and controlled. In real world cases where this scenario has been a contributory factor, this has not been the case. No sane pilot is going to deliberately fly into a stall unless training. $\endgroup$ – Simon Jan 22 '15 at 10:30
6
$\begingroup$

Any which the pilot thinks is more dangerous than the avoidance measures?

If you were driving at 100mph towards a wall which you reckoned had a 90% chance of a head-on collision, or could attempt to turn which had a 50% chance of throwing you off a cliff, which would you choose?

Any decision the pilot makes will be made on the basis of "Taking this action is a better option than not taking it".

The main weather which would likely cause this would be high enough levels of icing to be dangerous, or such severe turbulence that the pilot believes the plane will be "upset" enough to throw it out of the performance envelope. A combination of these may be present in a severe storm.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Thanks for your answer Jon. I assumed that there must be weather conditions which were so dangerous as to warrant a relatively risky course of action to avoid them. My interest was in what those conditions might be. A point which you have addressed in your last paragraph. Thanks again. $\endgroup$ – Tom Jan 22 '15 at 10:40
  • 2
    $\begingroup$ Possibly debatable, but not just turbulence. Severe up and down draughts can overwhelm an aircraft's ability to fly out of them. I see these as distinct from turbulence. $\endgroup$ – Simon Jan 22 '15 at 11:12
  • 1
    $\begingroup$ I see what you mean, in that Turbulence is chaotic whereas an updraught/downdraught system is on a larger scale so appears more "ordered", but both tend to come as part of a storm and are really just a type of turbulence (ie chaotic air movement). Also, attempting to climb out of or over an updraught is somewhat defeating the object.... either way, that seems to me to be arguing semantics, not principles. $\endgroup$ – Jon Story Jan 22 '15 at 11:18
  • 1
    $\begingroup$ @JonStory a very strong mountain can be very difficult to fly out of, especially in the Q corner. $\endgroup$ – rbp Jan 22 '15 at 12:46
  • 2
    $\begingroup$ Yes I definitely suggest not attempting to fly out of (or into, or through) any mountains $\endgroup$ – Jon Story Jan 24 '15 at 1:51
3
$\begingroup$

If you are talking about the AirAsia Crash, you may have some misapprehensions.

Normally, the problem with a storm is not "avoidance" once you are in it. The problem is keeping the aircraft flying in a stable configuration in the presence of violent winds. A powerful enough wind can literally turn a small aircraft upside down and backwards. Big Jets are much less susceptible, but can still be thrashed around violently in a storm. The pilot has to react to these upsets and keep flying. So, for example, if the winds put you into a stall, you have to recognize that and do what is necessary to keep air moving over the wings properly. Sometimes, the necessary actions might be completely unintuitive or different than what is normal. For example, if you are upside down, some of the controls work in reverse. If you do not know that, or are not practiced in flying upside down, then making that adjustment can be difficult.

A big problem in such circumstances is that in a storm you may not be able to see the horizon, so you have no visual reference for horizontal stability. Moreover, the artificial horizon, the instrument that tells you your attitude, has a lag, so what you are seeing on the instrument was the situation 1 or 2 seconds ago, whereas your current situation may have changed. In a spin or other rapidly changing flight condition the artificial horizon will be wobbling wildly and unpredictably and is less useful than normal in determining what the true attitude of the aircraft is. In a non-visible conditions this can lead to disorientation.

In the case of the AirAsia flight, the storm could have been putting them into a stall and the aircrew's reactions were not vigorous enough to correct it. Unless details are released on their control inputs and other data, we have no way of knowing. Also, they may have just taken the wrong actions. For example, if you are in normal stall you push the stick forwards to get the nose down, but if you are nose up and upside down, then you have to pull the stick backwards to get the nose down. Since you cannot see, it can be difficult to tell if you are upside down or not.

Also, sometimes a pilot will panic if they are losing altitude fast and pull back on the stick, a fatal reaction. Being inside a storm can be very frightening, even to pilots experienced with flying in them (such as meterological pilots), so panic is a real possibility. Once a person becomes panicked, their training goes right out the window and they can do very illogical things. I was once in aircraft with a high-hour CFI who panicked and he did some crazy things.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Thanks Tyler. I am not specifically referring to the AirAsia crash but rather am just interested in what kinds of weather conditions might be so unattractive as to make a pilot take extreme and possibly dangerous evasive action prior to entering those conditions. I'm interested because various 'fear of flying' resources reassure nervous fliers that modern passenger planes are built to withstand any weather conditions they are likely to encounter. This appears to be borne out by what you are saying but instead places the focus for crashes firmly on pilot error. $\endgroup$ – Tom Jan 22 '15 at 18:57
  • $\begingroup$ @Tom Well, first of all, icing can bring down any plane if it is severe enough, so pilot error is not the only factor. You never have to take "extreme" maneuvers to avoid a storm front, you just turn around. A plane can fly a lot faster than a storm. Most crashes involve a pilot making one or more mistakes, however, take into consideration that flying is very complicated, so expecting pilots to make perfect decisions without exception in a crisis is not realistic. $\endgroup$ – Tyler Durden Jan 22 '15 at 19:15
  • 1
    $\begingroup$ @NathanG Not only does the "AI" lag, it lags by different amounts depending on what you are doing. Lazy turns can cause really long lags. During upsets or gusty conditions it will rock back and forth and be very difficult to use. $\endgroup$ – Tyler Durden Jan 22 '15 at 20:35
  • 2
    $\begingroup$ That's news to me. Asked as a question to get more info: aviation.stackexchange.com/q/11924/136 $\endgroup$ – NathanG Jan 22 '15 at 22:51
  • 1
    $\begingroup$ QZ8501 was no Cessna 172 with 40 year old mechanical attitude indicator. It was A320 with AI fed by inertial reference system that can do pretty precise dead-reckoning and is used for flight controls and with alpha-limit that already saved many planes from crashing when they approached stall in heavy turbulence. If they really stalled, which we can't know for sure until NTSC publishes FDR data, there must have been a technical problem in addition to whatever mistake pilots may have made. $\endgroup$ – Jan Hudec Jan 23 '15 at 10:59
2
$\begingroup$

Realistically I can only think of one: downburst on final approach. In downburst the aircraft in relatively quick succession encounters headwind, downwind and tailwind. The increase of headwind causes increase in airspeed, but loss of kinetic energy due to increased drag. Then the downwind simply causes loss of altitude and finally the change from headwind to tailwind causes loss of airspeed that may be easily large enough to stall the aircraft.

If this dangerous phenomenon is encountered at low altitude (it only occurs at low altitude, too), the pilot must fly the aircraft at the edge of stall to preserve as much height as possible and hope the engines will be able to make up for the lost energy before he runs out of it. It is the reason Airbus introduced alpha-limit on A320 and alpha-protection already earlier. Alpha-limit limits angle of attack to just below stall when the side-stick is moved all the way aft and alpha-protection sets full power when a slightly lower angle of attack threshold is exceeded.

I suppose this is not what you meant. No, I can't think about anything at high altitude that would call for such aggressive manoeuvre. At altitude either you see the weather developing ahead for couple of minutes (with eyes or on weather radar) and avoid it using normal manoeuvres or you hit unexpected clear air turbulence and just react to it. And clear air turbulence is quite likely to make the plane loose altitude, hundreds of feet, rarely even couple thousand feet, but not gain it.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ Relevant reading, perhaps, for those interested: en.wikipedia.org/wiki/Delta_Air_Lines_Flight_191 $\endgroup$ – Jon Story Jan 22 '15 at 17:06
  • $\begingroup$ "And clear air turbulence is quite likely to make the plane loose altitude, even couple thousand feet, but not gain it." Unless it confuses the alpha protection, which then sends the plane into a steep, uncommanded climb. $\endgroup$ – reirab Jan 22 '15 at 19:15
  • $\begingroup$ "Even couple thousand feet" - excessive, surely? I've never heard of clear air turbulence causing more than a couple of hundred feet of altitude loss. A microburst can cause more of a drop, but that's not clear air tubulence $\endgroup$ – Jon Story Jan 24 '15 at 1:51
  • $\begingroup$ @JonStory: Usually hundreds, but thousands seem to happen occasionally. In this incident it was 7500 ft. That would include altitude lost while recovering from any attitude the turbulence got them in. $\endgroup$ – Jan Hudec Jan 26 '15 at 8:32
  • $\begingroup$ Tailwinds causing a loss of indicated airspeed is a myth $\endgroup$ – rbp Jan 26 '15 at 13:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.