Air Asia Crash Raises Questions For Pilots.


The search continues for the Digital Flight Data Recorder (DFDR) and Cockpit Voice Recorder (CVR) from the lost Air Asia flight 8501 and as that process drags on, speculation about the cause of the crash abounds.

Multiple news media sources advance abstract theories based more on the wide-open field of “what could happen” rather than what’s likely, serving only to blur the line between fact and fiction.

I won’t speculate on what happened to QZ 8501 because until the DFDR and CVR are recovered, transcribed and the recovered data analyzed, any theory advanced is just more noise in the media clamor aimed mostly at ratings rather than facts.

But, I can speak to what concerns me as the pilot of a modern, 160 seat airliner flying often in the same circumstances encountered by the lost flight. My goal in learning what the flight’s recorders report is simple: I want to know how to avoid a similar outcome.

With that in mind, here are my concerns. First, the slim margin between high speed and low speed limits at high altitude and the liabilities of each. Second, the problems presented by convective activity in crowded airspace. Finally, recovery from any inflight upset at altitude that may be encountered as a result of any or all of the above factors.

Early in any flight, the aircraft’s weight is the highest, limiting the ability of the aircraft to climb into the thinner air at higher altitude. As the flight progresses and fuel is consumed, the aircraft grows lighter and climb capability increases. Generally speaking, later in flight there are more habitable altitudes available due to weight constraints easing.

image

But don’t think that climbing is the only option for weather avoidance. Often enough, a descent is needed to avoid the top part of a storm, the anvil-shaped blow-off containing ice, high winds and turbulence. Equally as often, lower altitudes may turn out to have a smoother ride.

The other major climb restriction along frequently used jet routes is converging traffic. Aircraft flying opposing directions must be separated by a thousand feet vertically, so if I  want to climb to avoid weather, I have to nonetheless stay clear of oncoming traffic. The New York Post reported the incorrect statement that the air traffic controllers handling the Air Asia flight “made the fatal mistake” of denying the Air Asia’s pilot request for a higher altitude. The first job of air traffic control is to separate traffic, particularly converging nose to nose. Climbing through conflicted airspace–or granting clearance to do so–would more likely be a fatal mistake.

sunset 1

But there’s even more to the story: air traffic controllers respond to such requests in a more fluid fashion than the static “no” being implied by many media reports. In actual practice, for a climb or descent request, the denial would be more typically, “Unable climb, you have traffic on your nose,” or, “It’ll be 5 to 7 minutes before we can clear you higher,” or, “We can vector you off course so you can clear the airway and traffic and then climb,” or, “Unable in this sector, check with the next controller.” Regardless, there are other options to avoid weather.

If changing altitude is not an immediate option, lateral deviation is the next choice. But the same obstacles–weather and traffic–may limit that option as well.

So now, if vertical and lateral deviation isn’t immediately available, you must do your best to pick your way through the weather with radar, if possible, until one of those options comes available (again, at ATC denial isn’t final or permanent) or you’re clear of the weather.

Which brings us back to the margin between high and low speed limit. This is even more critical in convective weather, because turbulence can instantaneously bump your airspeed past either limit if there’s not enough leeway to either side of your cruise Mach.

The picture below shows a normal airspeed spread in cruise. Notice the speed tape on the left with the red and white stripe above and the yellow line below the airspeed number box. The hash marks represent 10 knots of airspeed. The red and black marker above the speed readout is called the chain, and it depicts the maximum speed limit for weight and altitude. The yellow line below the numbers is called the hook, and it marks the minimum speed required to keep flying.

adi 2

Turbulence, or more accurately, high altitude windshear, can bump you past either limit, or both, if there’s less than say, ten knots of slack, because moderate turbulence can cause swings closer to twenty knots; severe turbulence even more. Essentially, turbulence can instantly bump an aircraft out of its flight envelope.

In that case, the aircraft can depart controlled flight in a couple of different ways. The one that concerns me most is on the high end: if turbulence or any other factor pitched the nose down and the airspeed then climbed above the chain, the worst case is a phenomenon rarely discussed outside of the jet pilot community called “Mach tuck” that affects swept wing aircraft. Essentially, if you don’t immediately apply the proper corrective input, in a matter of seconds, recovery is beyond all means from the cockpit.

On the low speed side, if the wing stalls due to an airspeed below the hook, recovery is possible once the airspeed is regained. That takes altitude to regain, but normally can be done if a stall occurs at cruise altitude. But even that requires recognition and then the proper corrective control inputs, and Air France Flight 477 with three pilots in the cockpit entered a stall at cruise altitude but never identified the problem or applied the proper recovery inputs, resulting in a crash into the Atlantic that killed all aboard.

Bottom line: you need a wider spread between high and low speed limits in case of turbulence. If you can’t avoid turbulence and need to change altitude, you must assure a wide airspeed margin between limits to avoid being pushed by turbulence beyond either speed constraint. Here’s what the airspeed range looks like at high altitude:

adi 1

There’s very little tolerance for turbulence and any associated airspeed fluctuation.

In the worst case scenario, if the aircraft is pushed beyond its flight envelope to the extent that controlled flight is departed, a pilot must quickly and accurately recognize which situation is at hand, high or low speed buffet, then immediately apply the correct control input.

Problem is, they may initially look the same, and the correct remedy for one applied to the other severely worsens the situation. Specifically, if the aircraft begins a descent at a speed beyond the chain, the corrective action would be to deploy speed brakes, pull throttles to idle, apply back pressure to raise the nose, and I’d be ready to even lower the gear to add drag, even knowing that would likely result in gear doors being ripped off the aircraft.

If this recovery is not done early in the pitchdown, the result will be a dive with no chance of recovery.

If a low speed stall is encountered, the proper corrective action would be to add power and lower the nose until flying speed was recovered. But, if the high speed departure–also a pitch down and descent–was mistakenly interpreted to be a slow speed stall, applying the slow speed recovery to a high speed departure would be fatal.

The other way? If you mistakenly added drag and pulled back power in a slow speed stall? That would prolong the stall, but if the correct control input was eventually applied, the aircraft could recover, altitude permitting.

image

Adding the factors that make this vital task of discrimination difficult would be any associated systems failure and the physical effects of turbulence that can make instruments nearly impossible to read.

In any pitch down, if rapid and deep enough, can cause electrical failure due to generators failing at negative G-loads associated with the pitch down. Yes, back up controls and instruments exist, but recognizing the situation, taking corrective action and reading backup instruments also takes time and attention.

Pitot-static failure, one of the contributing causes in the Air France slow speed stall, can also be difficult to recognize in turbulence or in an electrical failure.

Regardless, the high speed situation must be correctly identified and recovery initiated in a matter of seconds. Both situations would be difficult to diagnose and both recoveries would be very challenging to perform in turbulence and with any other systems failure or complication. Both recoveries are time-sensitive and if not managed correctly, one recovery could induce the other stall. That is, too much drag and power reduction carried beyond the return from the high speed exceedence can induce a low speed stall, and too much nose down pitch and excess power from a slow speed recovery could push you through the high speed limit.

So here are my questions, which are those that will be asked by The QZ8501 accident investigation board. First what did the aircraft weigh and what was the speed margin at their cruise altitude and at the altitude they had requested? What type turbulence did they encounter and what speed and altitude excursions, if any, resulted? What collateral malfunctions, if any did they encounter? And finally, what departure from controlled flight, if any, occurred, and what remedial action, if any, was attempted?

These questions can only be answered by the DFDR and CVR and my interest–and that of every airline pilot–is mostly this: I want to know what exactly happened so as to be prepared in case I encounter the situation myself, and I want to know what they did in order to know what exactly I should or shouldn’t do.

Like pilots at all major US airlines, I get annual simulator training in exactly these scenarios, hands-on practice recovering from stalls and uncontrolled flight. Is that enough? Can we do that better?

Once the facts contained in the flight’s recorder are extracted and analyzed, we’ll have the answers to all of these questions, which will help us prevent a repeat of this disaster. Beyond that, speculation is just a sad, pointless part of unfortunate ratings-hungry media circus.

image

Advertisements

36 Responses to “Air Asia Crash Raises Questions For Pilots.”

  1. Bill Brandt Says:

    When a plane has an accident I can think of only one instance where the media speculation before recovery of the black box had any semblance of truth – the Air France 447.

    Most of the time – they are way off including input from “experts” opining when they have no facts – only speculation.

    I had always heard that some of the updrafts from these thunderstorms are powerful enough to shred these aluminum airplanes – true?

    Never stopped to think that with all the weird currents the aircraft might suddenly be outside its envelope.

    About the only thing we can be reasonably certain of was that the thunderstorm was a factor. And can’t they reach 60,000′?

    • Yes, I’ve flown in that part of the world and those storms are well up into the fifties. But I have to believe (hope?) that no one would blunder into the center of a storm, and that the periphery is survivable. Film at 11 …

      • That scenario makes for an interesting decision-making process. When a course deviation isn’t offered by ATC, at which point does one decide to deviate on their own authority? Something we all hope to avoid with timely communication.

      • Never. That would be reckless and could cause a midair rather than just turbulence. Bottom line, you don’t fly into a thunderstorm: as a last resort, you declare an emergency, exercise that emergency authority by doing what you believe is best, and ATC will move others out of your way.

      • So I’m wondering if this was a “last resort” scenario. Was there any doubt among the crew about the safe outcome of this flight? I’d like to know that as much as I’d like to know how to avoid the catastrophe.

      • That’s what my point is in this blog post: we need to know what happened to know what could or should be done in that situation. First, we need to know exactly what went on in the cockpit and only the DFDR and CVR will tell us that–soon, I hope.

    • Updrafts in strong thunderstorms can exceed 150 mph, and tops of supercells can push an “overshooting top” through the tropopause. Not to mention large hail to softball size suspended in the updraft. The most violent storms are found in latitudes outside the tropics, however any thunderstorm is a thermodynamic machine best avoided entirely.

  2. Thanks for the insights, Chris. Actually, you had me at “I won’t speculate…” and I wish more people wouldn’t.

  3. The slim margin you guys fly with makes my skin crawl. Even at FL510, the Gulfstream still has a comparatively huge margin compared with your aircraft.

  4. Bruce McNair Says:

    Thanks for your wise and calm discussion of what may have occurred. As a straight wing pilot I had no idea of the problems associated with those sexy sweep backs.
    Lessons will be learnt and procedures changed.
    Some good can come from tragedy.
    Thanks for all your posts, always enjoyable and enlightening
    Cheers

  5. The best article I’ve come across describing the possible problems pilots can encounter in that small window of flight. Thank you! Please write a follow up when you learn the facts and let us know what happened to that flight.

    (Sorry, typo in my email first time)

  6. roberthenryfischat Says:

    Reblogged this on robert's space and commented:
    because they always forget the trouble from the ground.pushbuttons.

  7. Frank Ch. Eigler Says:

    Chris, the only amendment I’d suggest to your post is to reduce the possible impression in readers’ minds that ATC is the final authority on routing. The pilot’s authority – emergency or otherwise – overrides ATC, which is something many laypeople don’t understand.

  8. Sarma HLN Says:

    Hi Chris,

    Very well explained and puts the spot light on how pilots face difficulties managing the flight. More importantly saving lives of the people in the aircraft including theirs.

    Thank you and have a great day. Saru

    Sent from my iPhone

    >

  9. Looks to me from the tail photos, there may have been an explosion in the APU compartment. metal peeled backwards, entire APU missing.perfect round hole with the APU was.

  10. Chris, you state, “Bottom line: you need a wider spread between high and low speed limits in case of turbulence.” But unless I missed it, I didn’t see you explain how that is actually achieved? Is it something you can control, as opposed to simply being a function of your altitude?

    • Weight and altitude are the two critical factors. Weight only changes with time, so altitude must be limited until weight allows sufficient pad on high and low speed limits. There’s nothing you can do to lower the weight, so the only other alternative is lateral deviation.

      • Frank Ch. Eigler Says:

        “There’s nothing you can do to lower the weight…”

        Well, management frowns on dumping fuel or cargo or passengers …

      • Management? In a weather situation, in the cockpit? Not even close. And there’s no fuel dump capability anyway.

  11. Damon Hynes Says:

    With this and some of you other posts, it seems like you’re always within sniffing distance of the coffin corner, although I’m sure as a professional that 50 knot window probably looks huge. Between this and a recent a.net discussion of TATL eastbounds getting caught in the jet and doing 750+ MPH, it’s a wonder how anything stays aloft!

  12. I was surprised at your comment that the greatest turbulence in a storm is at anvil level. I would have guessed flying through the updraft or updraft/downdraft interface would not only be more turbulent but also more prone to large variations in wind shear. If you simply HAVE to fly through a thunderstorm, which area would most predictable and “safest”? Or is this a judgment call based on the individual storm and it’s radar presentation?

    • I didn’t mean to suggest the worst turbulence is from the anvil, just that a descent, rather than climb might be called for to avoid the anvil. I don’t think it’s ever a good idea to go through a thunderstorm.

  13. photoncourier Says:

    I believe some military jets deploy speedbrakes automatically when a limiting mach number is exceeded,,,some versons of the MIG-15 did this, and some of the Czech jet trainers…do any airliners have this feature? Is there a reason why this would be a bad idea?

    • Yes–any automatic deployment of a drag device comes with the unacceptably dangerous possibility of a sensor malfunction and a subsequent inadvertent deployment. During low speed flight on takeoff or landing, that would be a disaster.

  14. Thanks for the information Captain Manno. I always wondered about the coffin corner and how challenging it may be to stay within the margin of safety. I had no idea about the coffin corner until after I earned my PPL. I’m now working on the IFR. And now, I’m more fearful of flying in a jet! Go figure, sometimes it’s best not to know. I’m glad to know that changing altitude increases your airspeed safety envelope. I hope whichever captain I fly with has this ingrained in her/his head. This is the first blog I’ve read of yours after I heard it mentioned on the Airline Pilot Guy podcast. I like your writing style, very sharp, cuts right through the nonsense! Thank you Captain Manno.

    • Not really. Air France experienced pitot static failure and false instrument reading and erroneous warnings. QZ5810 had no pitot static failure and the warnings were accurate.

      • Frank Ch. Eigler Says:

        (The initial cause of the upsets may well have been different, but I was referring to the apparent responses of both sets of pilots, namely stalling from cruise into the water.)

      • Re: Air Asia, I’ve always said: start-up carrier, poor regulatory oversight, pilots with low time in high tech FMS, state-of-the-art jets = disaster. That’s the essence of the SFO Asiana at SFO too. There will be more.

  15. Sagegreen Says:

    Chris – great article as ever. You don’t explain why is “mach tuck” (as a possible explanation) is unrecoverable? Is it because the aircraft’s speed will keep increasing, with no means of slowing it? Thx.

    • There are many factors, but the big one is that eventually the Mach tuck exceeds the elevator authority. Then, no way to raise the nose.

      • Sagegreen Says:

        On your speed tape pic, what’s the significance of the series of red squares? The lower “hook” is self evident as, presumably, the airspeed you never go below. The upper squares signify…what?

  16. HI Chris, I recently came across this site and for the past couple of days I’ve truly enjoyed reading all that you’ve written here. It really makes me wish, even more so than I already do now, that I had gotten into aviation earlier in my life – I’m 30 now and not sure how that would work out for me, but that’s what research is for.

    The part of your article where you stated that negative g’s will cause the aircraft’s generators to fail has me wondering about why that would happen. What component is it exactly that the g-forces act upon to cause a failure?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: