07/10/2013 10:38 am ET Updated Sep 09, 2013

Asiana Airlines Flight 214 and the Breaching of Stabilized Approach Criteria


Before embarking on a little more factual analysis of Flight 214's accident -- I wish to acknowledge the sterling work that the National Transport Safety Board (NTSB) has conducted in their initial response to the Asiana Airlines Flight 214 accident at SFO on July 6, 2013. Having personally experienced the tsunami of information that a Board of Inquiry has to consider in my previous career, balanced with the unrelenting appetite of the media, Chairwoman Hersman and her investigation team's efforts are commendable.

The Airline Pilots Association (ALPA) on the other hand has said it is "stunned" that such "unprecedented" volumes of information have been divulged by the NTSB to the public as "it encourages wild speculation." Conjecture, whilst I do not agree with it, is always going to occur after accidents of this nature. In the contemporary media environment, I would contend that calculated and factual releases of key data points actually serve to tame the speculation and generate more accurate discourse, rather than detract from debate, in the days after an accident.

The NTSB should, however, be cautious about statements that indicate opinion at this early juncture avoiding where possible soundbites such as "they were very slow in this critical phase of flight." Stating the speeds of the aircraft on approach and the target speed over the threshold (VRef) only, should avoid subjectivity.

The approach data and in particular the speed of the aircraft as it descended on its flight path unveils key clues into the causes of the accident. In my experience, if an aircraft's speed drops, uncorrected, well below the calculated VRef near to the ground, then a stall will develop causing a catastrophic loss of lift.

As an aircraft descends on an approach, the HP, monitored by the crew, is required to be within a tolerable window achieving certain speed, engine, directional, rate of decent and aircraft configuration criteria. When such conditions are established, the approach is considered by the CAA to be 'stabilized' and in a safe configuration to land. The criteria, assessed as 'gates', are prescribed by individual airlines in accordance with their Standard Operating Procedures (SOPs). Airbus have defined the 'Elements of a Stabilized Approach' here.

If any of the conditions are breached and not corrected immediately, then the approach is defined as 'de-stabilized' and a 'go around' is to be initiated by the Handling Pilot (HP). From experience, a 'go around' can be called by any member of the crew, and if the HP is not content with the order, then questions should only be asked once the aircraft is positively climbing away from the ground and in a safe flight configuration.

We know from Chairwoman Hersman that the VRef for Flight 214 landing on SFO's Runway 28L was 137 knots. The speeds from the Flight Data Recorder (FDR) at various heights on the approach were also briefed at the NTSB press briefing on July 8, 2013. Critically, at some point in between 1,000ft (54 seconds before impact) and 500ft (34 seconds before impact), the approach speed drops permanently below the target approach speed (VApp = VRef + 5 knots = 142 knots), and outside of the 'stabilized' window. During this phase of the approach, and with the information available from the FDR, a 'go around' should have been initiated by the HP or directed by a member of the crew of Flight 214.

How the aircraft arrived in a destabilized approach configuration will be for the NTSB to determine. Empirical contributory factors from previous accidents include: weather, fatigue, poor crew resource management, technical difficulties, expeditious approach requests by Air Traffic Control, lack of experience, unfamiliarity with the airfield and on type, and the physical condition of the pilots and crew.

The list is not exhaustive but insinuating that any one of these factors might have contributed to the Asiana Airlines accident would be purely speculative at this stage. We should also not underestimate the difficulty and pressures that Chairwoman Hersman and her team will be facing -- so far a exemplary performance all round by the NTSB.