The Downing Death Knell for Air France 447
A new Thales Pitot probe Part Number (P/N) C16195BA has been designed
which improves A320 aeroplane airspeed indication behaviour in heavy rain
conditions. This same pitot probe standard has been made available as
optional installation on A330/A340 aeroplanes, and although this has shown
an improvement over the previous P/N C16195AA standard, it has not yet
demonstrated the same level of robustness to withstand high-altitude ice
crystals as the Goodrich P/N 0851HL probe. At this time, no other pitot probes
are approved for installation on the A330/A340 family of aeroplanes.
Airspeed discrepancies may lead in particular to disconnection of the autopilot and/
or auto-thrust functions, and reversion to Flight Control Alternate law,
which would cause an increase of pilot workload. Depending on the prevailing
aeroplane altitude and weather, this condition, if not corrected, could result in
reduced control of the aeroplane.
fromEASA PAD No : 09-099
NOTIFICATION OF A PROPOSAL TO ISSUE AN AIRWORTHINESS DIRECTIVE
IMHO the pitot problem is all to do with heating and heat
dissipation rates prevailing in particular environmentals.
i.e. the thermodynamic calorifics of the particular scenario. I get
the impression that this phenomenon was poorly understood at the
design stage (by Thales or Airbus)..... and maybe even now.
Pitot tubes on aircraft commonly have incorporated heating elements called (generically) pitot heat to prevent the tube (aka "probe") from becoming clogged with ice. The failure of these systems can have catastrophic consequences, as in the case of Austral Líneas Aéreas Flight 2553, Northwest Orient Airlines Flight 6231, AeroPeru Flight 603 (blocked static port), and of one X-31. At lower levels (and consequent higher ambient temperatures) pitot heating is sufficient to stop ice forming on the pitot tube and blocking air entry. On the ground the pitot heat is normally inhibited by ground/air sensing (to stop the pitot heat from burning out)
At higher levels the threat to the vital free passage of air into the pitot system is a little more complex. Think in terms of BTU's/hour or calories/hour. If the pitot heating provided at higher altitudes is (say) 1500x calories per hour and the diabatic process (due to the cooling airflow) sucks heat away at only 1300x cals/hr, then net heating is taking place and the pitot tube will remain hot enough to melt ice (or stop it forming). x is used here as a multiplier because quite evidently we're talking millions of calories/hr. The Wikipedia article at this link discusses the difference between diabatic and adiabatic and explains the terms isothermal and entropy in this context. As you will see from the differential calculus at that link, it's no simple process as it's not as straightforward as it might seem. There's latent heat of evaporation to also take into account once moisture (gaseous or liquid or solid) enters the picture. Even if variable heating provisions are made (in anticipation of the varying thermodynamic potentials), it all depends upon where the controlling sensor is located. The ability of a singular sensor to accommodate all the variables is also at question here.
To explain. At height, in cirrocumulus cloud, the ice crystals will tend to accrete (which is different to rain-water or snow forming ice) in the pitot entry. The calorific gain and loss factors at the pitot tube then become quite different. The pitot heating can become overpowered allowing the ice crystals to be melted but then re-form within the tube and ice it up further back (even if only partially)... once downstream of the heating element. It's important to note (http://tinyurl.com/ytcwpv) that there's no through-flow in a pitot system i.e. the fluid pressure is measured as the encapsulated air stagnates. Normally any ingested water will depart (vent to atmosphere) via the drain hole just prior to the 90 degree curve in the aft pitot. That pitot feature should always be checked as being clear on pre-flights as it's quite a small diameter escape orifice. It can easily be blocked by insects and/or ice - and presumably, a congestion of ice crystals.
My guess is that the higher altitude thermodynamic characteristics of the Thale pitots have always been essentially flawed, Thales having made no provision for detecting (or testing for or compensating for) any heating deficiencies caused by these vastly differing environmentals. Needless to say, in a triplex system the value of redundancy is lost when left and right pitots are being simultaneously subjected to these identical, yet compromising, conditions. The dispensation for the central under-nose pitot is probably being made due to it being located in a slightly different thermo- (and aero-)dynamic locale and it feeding the standby ISIS system (for the backup flight instruments).
Why hasn't this phenomenon been more common in earlier A330 operations? Modern airliners have the capability of dialling up different operating emphases by way of Flight Management Modes. These include Minimum Servicing Costs/flt hr, best air nautical miles per pound of fuel (ANMPP), minimal flight-crew costs (if crews are paid by the flight hour) or minimum time enroute (if aircraft are required for tight turnarounds for the next sector to beat a curfew - for instance). As airlines hedge fuel costs and experience light load factors, they're generally going for max economy and crews dial up FMS modes for cruise that achieve best ANMPP. The implications here are for a higher altitude earlier and a corresponding lower cruise speed approximating 110% of long range cruise. This change in operating characteristics and environment means a subtle change in the thermodynamics at height. The "soak" time spent subject to ice crystal hazards is increased, as is the aircraft's susceptibility.
The FAA and EADS, UKCAA etc simply accepted and certified the Thales pitots on their manufacturers specs and have done nothing useful about hardware rectifications as a result of the plethora of incidents leading up to Air France 447's loss of control. The pre-AF447 advice to flight crews amounted to not much more than "You all be careful now you hear". Of course, up near coffin corner, when in turbulence at night in an aircraft automatically subjected to a degraded flight control regime once the computers become perplexed, sooner or later a flight crew was destined to be surprised and fumble the ball. AF447 lost autopilot, lost a number of speed-dependent systems, lost control and ended up in an unrecoverable unusual attitude.... all in short order. In the absence of recorder evidence, who's to know whether it was a deep stall, flat spin or some unknown extrapolated extension of a degraded Fly-by-wire system taken well beyond its acceptable boundaries?
What is clear is that no manufacturer, operator or regulator ever bothered examining the possible and plausible ramifications of a known failure condition that would simultaneously nullify redundancies, cripple safety systems and cross border-lines for human factors limitations - until an accident clarified the existence of these critical interrelationships.