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Another Case of Pitot Probes Blocked by Ice

Tue, Feb 15, 2011 — David Evans

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The speeds indicated by the three pitot tubes varied by 30 knots, from 230 to 260 knots. The captain took control from an inexperienced first officer and safely diverted the planned flight of the A320 from Nuremburg to London, landing the airplane and its 59 passengers and crew at Nuremburg.

The serious incident parallels the fatal crash in the South Atlantic of Air France flight 147, a larger A330-200, killing all 228 aboard. (See Air Safety Journal, August 2009, “Prompted by Crash, Airworthiness Directive Issued on Pitot Probes,” and current issue, “Significant Regulatory & Related Activity”)

The incident Airbus A320 and the accident A330 have two things in common: (1) highly computerized fly-by-wire (FBW) flight controls, and (2) pitot probes supplied by Thales.

In both events, the supposed triple redundancy of (1) and (2) were compromised.

Airbus has been prompted to issue a bulletin, FCOM 824/1 – Erroneous Airspeed/Altitude Indications, advising flight crews of symptoms and actions to take. The FCOM says in part:

Failure Case Consequences
Water accumulated due to heavy rain.Drain holes unobstructed. Transient speed drop until water drains.IAS [indicated air speed] fluctuations.

IAS step drop and gradual return to normal.

Water accumulated due to heavy rain.Drain holes obstructed. Permanent speed drop.
Ice accretion due to pitot heat failure or transient pitot blocked due to severe icing.Unobstructed drain holes. Total pressure leaks towards static pressure IAS drop until obstruction cleared/fluctuation if transient erratic ATHR [auto-thrust] if transient.
Ice accretion due to pitot heat failure or pitot obstruction due to foreign objects.Obstructed drain holes. Total pressure blocked Constant IAS in level flight until obstruction cleared In climb IAS increases In descent IAS decreases Abnormal AP/FD/ATHR behavior :

  1. AP/FD [autopilot/flight director] pitch up in OPN CLB to hold target IAS.
  2. AP/FD pitch down in OPN DES [descent] to hold target IAS

Airbus advises crews that “in very extreme circumstances it may happen that two, or even all three ADR may provide identical and erroneous data.” The autopilot, flight director and auto-thrust should be disconnected and the target pitch attitude and thrust setting should be flown.

This tidy guidance assumes that the crew has not been surprised by, say, the sudden pitch-up of the aircraft and its departure from controlled flight – the Air France flight 447 case.

Things had not gotten that bad in the case of the A320 bound for London when the speed readings went bonkers at 11,000 feet. The incident was investigated by the German Federal Bureau of Aircraft Accident Investigation (BFU).

Salient tid-bits from the BFU report of May 2010 indicate what happened:

“During the night [at Nuremburg] there had been a fall of approx. 20 cm [7.8 inches] new snow…”

“The wings and tailplane had been de-iced prior to the flight. There had been no intention to de-ice the fuselage.” (The three supposedly redundant pitot probes are mounted on the forward fuselage.)

After the landing in Frankfurt, ice was still visible on the aircraft nose over the static ports and pitot probes.

“It is therefore quite possible that water melted from snow or ice into the apertures for the Pitot Probes and Static Ports.”

“There was a sudden loud bang from the right hand side of the aircraft … followed … by ‘ADR1 [air data reference], ADR2 and ADR3 FAULT’. There was simultaneous failure of both Autopilots, Auto Thrust and both Flight Directors. The control system mode switched to Direct Law. All the protective mechanisms had disappeared from the airspeed indicator in the Primary Flight Display (PFD).” (The fatal mistake of the Air France pilots was to re-engage the autopilot.)

Snow and residual ice on the aircraft nose below the cockpit windscreen after the landing in Frankfurt.

“From this point on the system was unable to undertake any plausibility checks because all three ADRs indicated faulty operation and the system was no longer able to identify which ADR was operating correctly.”

“The different speed indications were very probably due to impurities in the Pitot Static System.”

When ice blocks the pitot probe speed readings can be varying and below what the aircraft is actually flying

The Thales-supplied ADR2 was replaced by maintenance personnel, as it had been acting up the day before the incident. Quite surprisingly, according to the report, no test of the pneumatic tubing or pipes was made for water contamination. It is apparently the norm to assume that an electronics black-box swap-out would address all such failures. This belief would appear to be, in and of itself, a further serious failure.

The BFU had no recommendations. Although Airbus had recommended replacement of the Thales units with probes supplied by Goodrich on the A330/A340, no similar recommendations were issued for the A319, A320, A321 models. The Goodrich probe is less susceptible to ice contamination.

The BFU did not recommend a two-stage heating arrangement in the pitot probe to counter the effects of ice blockage.

The BFU did not recommend a change to the de-icing procedure before takeoff so that the fuselage and its ice-sensitive components are thoroughly doused in de-icing fluid.

The BFU did not ascertain if pitot probe contamination had been a scenario in the Failure Mode Effects Analysis (FMEA) of the speed readings during system certification.

An investigation bereft of any recommendations seems rather pointless. For every incident and accident, there are corrective measures.


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