The remains of an insect in a pitot tube of a British Airways jetliner may well provide a clue to what happened to Air France Flight 447 when it experienced a loss of control and crashed into the Atlantic in June 2009, killing all 228 persons aboard.
The insight comes from a serious incident report recently issued by the UK’s Air Accidents Investigation Branch (AAIB). For “beetle,” read “progressive super-cooled icing of all pitot heads” in cirrostratus/cirrocumulous clouds and you have the AF 447 pitch-up precursor to loss of control.
After takeoff from Accra, Ghana, as the British Airways B757 climbed, the captain’s computed airspeed began to rise “because the pitot pressure, trapped in the blocked pitot tube, remained constant whilst the static pressure decreased with altitude,” according to the AAIB bulletin.
The bulletin goes on to say:
“The FMCs [Flight Management Computers] use data from the left ADC [air data computer] unless a failure has been detected, in which case the FMCs use data from the right ADC. The ADC may not determine a blockage in the pitot system to be a system failure [emphasis added] …
“There is no specific guidance in the SOPs [standard operating procedures] on what to do should the ASIs [air speed indicators] disagree.”
Because the FMCs were using the left ADC data, they sensed an overspeed condition and provided a pitch-up command to reduce the airspeed. The co-pilot was concerned about the aircraft’s behavior and, after several urgent verbal prompts to the captain, pushed the control column forward. The captain, uncertain as to what was failing, believed that a stick-pusher had activated (a footnote to the bulletin indicated that the B757 they were flying is not fitted with a stick pusher). The captain disengaged the autopilot and lowered the aircraft’s nose then, noticing that the co-pilot had control, handed over control to him while he transmitted a MAYDAY.
An uneventful approach and landing were made.
Following the landing, maintenance technicians found the remains of a “beetle-like creature” in the left-hand pitot system. British Airways now places covers on its pitot tubes when the aircraft is on the ground during long turnarounds, a blocked pitot tube is part of simulator recurrent training, and pilots are advised to reject takeoff if the problem of a blocked pitot is recognized at speeds below 80 knots. The use of pitot covers is a good idea, and may be appropriate in short turnarounds, as well. It might also be useful to establish pilot responsibility to remove and stow the covers.
The AAIB bulletin noted that “There were times during this flight where the flight crew was confused as to what was happening.” Confusion is easy to create once raw data (like pressure sensor inputs) are suddenly compromised, or are allowed to trigger a follow-on event affecting controllability.
The AAIB bulletin reads like a direct replay of the Birgenair B757 crash of 1996, just minutes after takeoff from the Dominican Republic. The report into the cause of that accident stated, “Confusion of the flight crew occurred due to the erroneous indication of an increase in speed.” This report hinted at the possibility of a pitot tube clogged by an insect, as the airplane had been on the ground for a long turnaround.
The remains a mile down in the water of the Birgenair B757, downed by a blocked pitot tube and resulting crew confusion.
Now consider the Air France A330 flying from Rio de Janeiro to Paris at 37,000 feet, encountering severe thunderstorms along the way. To be sure, ice crystals tend not to stick to airplane surfaces, even above 40,000 feet. However, towering thunderstorm clouds produce “supercooled water” whose temperature is below freezing but turns to ice when it comes in contact with the airplane’s surfaces. For pilots and passengers alike, such clouds also generate frightful up and down drafts.
If pitot tube heating was insufficient to deal with icing, and the tubes were blocked, the airspeed would read lower than it really was at the time. [See Air Safety Journal, ‘Prompted by Crash, Airworthiness Directive Issued on Pitot Probes’] Those low readings would tell the computers running the plane to throttle up the engines. Note that only the “tubes” on the pitots are heated. Ice (impact crystals) can be melted at the heated portion but then cumulatively run back into the system, where it can refreeze, constricting the airflow and producing (effectively) a lower speed reading.
The effect of ice blocking the pitot is a false speed reading.
That lower speed reading is then interpreted by the system as a dictate to “power up” the engines. The net effect is an actual aerodynamic speed that is verging upon – or exceeding –the maximum Mach of the aircraft’s operating envelope.
If all the A330’s pitot tubes were blocked, the faulty readings could be accepted by the aircraft’s computers as accurate. The airplane would increase speed, experience what is euphemistically called a “departure from controlled flight,” and the pilots would suddenly be faced with a severe nose-down pitch compounding the excess speed problem.
One suspects the following scenario on that “dark and stormy night” over the Atlantic:
— The pilots had no way to tell they were going way too fast.
— Pilot confusion can be exacerbated considerably by ADIRU (air data inertial reference unit) associated alarms and ECAM (electronic caution alert module) messages.
— That two or three pitots can harmoniously fail (i.e. not produce any “disagreement”) and consequently allowing autothrottle to incrementally compensate for what the system sees as a loss of airspeed.
— By the time the system is so far out of whack that the autopilot disconnects itself, the auto-trim will be so far in error (out of trim but the flight level being “held” by the autopilot’s barometric hold function) that the pilot will be faced with an instant pitchup.
— Subsequent to pitchup, the pilot will see the low indicated airspeed (which is false), suspect a stall and react by increasing power while diving to regain airspeed. This reaction will unwittingly put the pilots in a loss of control situation that the A330’s fly-by-wire (FBW) system will be powerless to oppose.
— Faced with a sudden life or death situation, amidst the confusion CRM (crew resource management) may break down, leading to an uncoordinated response.
With faulty airspeed and no prior experience at hand-flying at those altitudes in Alternate Law (which limit the pilot’s control inputs so as not to overstress the airframe), nor at speeds well in excess of the aircraft’s operating envelope, loss of control was inevitable.
A European Aviation Safety Agency (EASA) briefing of September 2007 talked about airspeed fluctuation events in skies over Europe as a result of icing or heavy rain conditions, and the risk of “misleading airspeed indication on two or three airspeed indication systems.” The misleading airspeeds when the pitot is blocked should be considered hazardous, according to the EASA briefing, and certification of pitot probes should be upgraded in light of these findings.
The AAIB bulletin documenting the mayhem caused by an insect carcass shows what can happen when ice blocks the pitots, too.