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.
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.
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.
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:
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.
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.
No comments:
Post a Comment