Saturday, January 10, 2015

Comparing QZ8501 to Air France 447

On December 30, 2014 two days after the loss of Air Asia Indonesia (QZ) flight 8501, CNN asked me to provide a comparison between QZ 8501 and Air France 447.
The following was published 12/31/14

On Sunday, all contact with Air Asia flight 8501 was lost over the Java Sea as a wide area of thunderstorms covered the area. The discovery of floating debris on Tuesday about 100 miles from its last known position in combination with an analysis of ocean currents will give investigators clues where to search for the remainder of the aircraft. From its cruise altitude, the airplane’s gliding distance would also be about 100 miles, but consider that for the debris to drift that same 100 miles it would only take a drift rate of 2 knots, yielding a wide range of possibilities as to  the nature of the aircraft’s descent to the water below.

Many parallels between Air Asia 8501 and Air France 447 in June, 2009 are obvious. Both aircraft were lost in thunderstorm areas of the Intertropical Convergence Zone (ITCZ). Both were found within a few miles of its last known cruise altitude position, both were sophisticated fly-by-wire Airbus aircraft (though different models), and both crashed at sea.

While flying into a thunderstorm is always to be avoided, it not likely the sole cause of the accident.
The weather in the ITCZ has some unique qualities compared to your average thunderstorm over land. The storms are driven by the convergence of airflow patterns between the northern and southern hemispheres of the Earth in addition to the usual factors of warm moist air and unstable atmospheric conditions. The height of the stratosphere –- which tends to put a cap on the height of thunderstorm growth, and averages about 35,000 feet over the mid latitudes (such as that of mainland USA), reaches to 50,000 feet or more, providing for the growth of thunderstorms to great heights and accompanying intensity. These features can lead to some unusual conditions within those storms, making the proper assessment of them with airborne weather radar more difficult.

In the aftermath of the Air France crash significant emphasis has been made in pilot training on the prevention and recovery from similar scenarios. I would say that few pilots, especially of Airbus aircraft would be unaware of AF447’s lessons, almost certainly one with the reported experience of QZ8501’s captain.

There is a recent development however that relates to Airbus A320 series aircraft. A December 10, 2104 Airworthiness Directive (AD 2014-25-51) describes how control of the aircraft could be lost in flight as a consequence of icing of the angle-of-attack probes and an interaction with the airplane's stall protection function. Those probes act like small weather vanes on the side of the aircraft and measure the angle at which the airplane moves through the air--the angle of attack. If the angle is too high the air can no longer flow smoothly around the wings, resulting in an aerodynamic stall. The acceptable range of angles of attack is fairly small, and gets considerably smaller at higher speeds, such as cruise speed.

Simply put, depending on the position of the angle-of-attack probes when freezing occurs and the subsequent speed of the aircraft, the system may be fooled into thinking that the aircraft is approaching a stalled condition-even when it isn’t. In response, the airplanes stall protections pitch the aircraft’s nose down to recover. This erroneous pitch down cannot be overridden by the pilots unless an emergency procedure in the Airworthiness Directive is followed. All pilots flying this model airplane should be aware of this.

The procedure instructs the pilots to shut down two of the three air data computers to render the usual stall protection inoperative an allow recovery of the aircraft. Of course, there is no way, at this stage of the investigation, to know if this played a part but investigators will certainly be looking for evidence of this phenomenon.

Another obvious question is the apparently lack of transmitted position and altitude data after its last known position in cruise. This data is transmitted throughout the flight by a system known as ADS-B (Automatic Dependent Surveillance-Broadcast). This system transmits the airplane’s position and other basic data to ground stations. Though its position is GPS satellite derived, it is not transmitted to satellites, only to ground stations – so the range to the nearest station is a factor.

The apparent sudden loss of this data at cruise could be explained by failures in flight such as an electrical failure, in-flight breakup of the aircraft, or the pilots switching off required data to operate the system such that outlined in the emergency procedure above. However, it could also be that the aircraft simply flew out of range of the ground stations. Flight tracking websites indicate that this routinely occurs in the general area where QZ8501’s last ADS-B transmission was made. I think that is the most likely cause of the end of the data stream and is not necessarily an indication of catastrophic failure in flight.

In the case of Air France 447, the aircraft came down in the Atlantic Ocean where the sea depth exceeded 12,000 feet. While some floating wreckage and a of number of bodies were discovered within a few days on the surface, the extreme depth and rough terrain on the ocean bottom delayed discovery of the remainder of the aircraft and recovery of the flight recorders for two years.

Fortunately, the 100 foot depth of the Java Sea in the area where evidence of QZ8501 was found will almost certainly result in the relatively rapid location of the aircraft and recovery of the two flight recorders. Consideration of ocean currents during the two days between the aircraft’s disappearance and the discovery of floating debris will help lead investigators find  the remainder of the aircraft and its passengers. We should not be subjected to long period of uncertainty such as with AF447 or the continuing lack of information on MH370.

In the aftermath of the Air France crash significant emphasis has been made in pilot training on the prevention and recovery from similar scenarios. I would say that few pilots, especially of Airbus aircraft would be unaware of AF447’s lessons, almost certainly one with the reported experience of QZ8501’s captain.

While any accident investigation will take months to complete, I would expect more information to be available as the search and recovery continues. Clues from the way in which airplane parts were damaged on impact, the flight data and voice recorder contents will provide answers. But like any aircraft accident, the cause is likely to be the result of a chain of events and conditions, the absence of any one of which would have avoided this tragic accident. At this time we can only guess what some of those events and conditions are.

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