The Air France accident has had its repercussions as will the spate of over-runs later as airline training departments craft their next Checks. Unreliable Airspeed has emerged as a feature on this seasons ATQP offering.

The blockage, loss or failure of pitot and static systems or air data computers has caused several accidents across the years. Current theories on the loss of AF447 GRU-CDG 1st June 2009 focus on these systems.

A brief description of the issue for anyone not familiar with the nature of the problem, and at systems level it is a complex one and specific to type so I will not comment on the mechanics other than generally.
I hope this gives an insight and overview – it certainly isn’t designed to be a flying lesson “your honour’.

Modern airliners are designed to be flown through a Flight Control System that has many components. At the hub of this system is the autopilot, a device that takes inputs from an air-data system which is fed from sensors that provide airspeed (pitot) and static (prevailing corrected ambient) pressure. If these (twin if not triple) normally reliable and accurate sources provide false information to either pilots flight instruments they can seriously affect the job the autopilot is normally tasked to do, that is, fly the aircraft.
I say ‘normally tasked to do’ because the autopilot flies the aircraft for the majority of the time aboard modern airliners of all flavours. Notwithstanding that, it is the prime responsibility of the pilots to maintain control of their aircraft – fly a safe flight path. Obvious to the dear reader perhaps, but a pivotal statement when the autopilot cashes in its chips… and it does just that if it receives erroneous feeds from the air data system. It doesn’t always tell you it has done so either, it sometimes valiantly hangs in there adding to the ‘fun’ as it tries its best for you.

At the point of recognition.
Deviations from normal, controlled flight can be insidious leading a crew to the point where a rapidly developing control related situation can be delivered robbing them of time to mentally adjust. They are most likely to occur just after takeoff, within severe icing conditions or when systems failures combine with events. They seldom if ever happen in the cruise, in clear air and when enjoying a stable state of flight. They are very, very rare given the number of aircraft flying and the hours being flown.

Initial symptoms can be insidious, airspeed rising or falling just outside the normal rate adding an progressively increasing increment on what is being expected. The autopilot responds to the symptoms in a perfectly normal and logical way. The only thing missing from the picture is a reason for these initial perhaps gentle excursions. Distraction with normal flight deck tasks are always present, ‘helpfully’ high workload segments of any flight and prime time for these situation to occur.
Flight path monitoring is the primary task during departure but getting behind the work volume causes its own problems so pilots prioritise and manage their attention focus on the run shaped around their normal working habit patterns. Remember this problem is very rare which means anticipation is mostly suppressed – shock probably always accompanies the event…

First recognised symptoms might be the unexplained excursions from the normal – a remark from the handling pilot that something is wrong, doesn’t figure, is unusual. Out of place audio warnings may occur along with conflicting information from the flight instruments. Established normal patterns of instrument scan compound the confusion. Pilots in the past have experienced serious difficulty interpreting and ordering the information of them given that they use most of what they see and believe it most of the time. Analysis of simulator exercises (NASA etc) and experience coupled with the losses outlined in the links above attest to this.

When diversions from normal flight/unreliable airspeed indications are picked up by the pilots it is essential that they intervene and follow their memory item drills for their aircraft type. They will almost certainly include disengaging the autopilot, flight director and auto-throttle (for they know not what they do) and manually fly the aircraft to a stable state. That sounds simple doesn’t it?

Manifestation and recovery – phase 1
Obviously if control has been lost it must be quickly regained. An immediate focus on the fundamentals of flight, attitude versus power and the required performance from the aircraft is vital. Returning the aircraft to a stable situation is the top priority and a pre-knowledge of ballpark pitch attitude and thrust figures for climb, cruise and lower level flight are what you might understate as ‘handy to have’ bits of information.
When safe flightpath has been secured analysis and fault finding can occur. If this situation isn’t taken quickly and firmly in hand then an unstable train of events can follow, possibly leading to the loss of the aircraft.

The last paragraph may seem strikingly obvious and a surprising read. How a train of events sets up to cause the loss of an aircraft and many lives takes a little understanding if you have never been the victim of sensory overload, the first effect that attempts to drown the essential reasoned response.

Recovery – phase 2
Having regained control of the aircraft Standard Operating Procedures now prevail. Both pilot’s heart rate may have hit their peak during phase 1 but dropped away from arrest levels as control was regained. Preparing for the next phases of flight dominates with focused management from the crew; checklists and analysis aid fault finding to the point where either systems are restored or the aircraft can be safely flown to a safe landing with unserviceable systems. The latter phase is very demanding but becomes a good deal easier with practice.

The major challenge is without question the management of the flight, the technical challenges are fairly black and white. All the performance information you require is contained within the aircraft documentation. The Quick Reference Handbook (QRH – sometime anything but Quick) has the detail, training provides the context.

Boeing and Airbus provide aircraft systems based around their particular philosophy. The objective behind the tables below is to provide known performance in the absence of instrument indications based on pitch and power settings. If you render this down is equates to a safe angle of attack for given configurations.

I believe Airbus thoughtfully provide a direct reading of angle of attack in some instrument display modes. The Boeing supplied Primary Flight Display (PFD – artificial horizon in its early incarnation) has an inertially derived Flight Path Vector symbol (FPV) from which the knowledgeable (using pitch information) can derive current angle of attack – or something close and safe. That last statement is crucial but this post cannot be a flying lesson for obvious reasons.

We may never know what caused the loss of AF477 for a variety of reasons. Complex, highly automated systems with inhibitors to pilot action built into them may be contributory in situations like these. Familiarity through training and a standardised approach to the problem can and does lead to a successful outcome when imaginative scenarios are flown… in the simulator. The best place by far to experience your average nightmare.

Tags

Air Safety, Flying General, Human Factors