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In the incident examined in this video the pilots experienced an unreliable airspeed indication due to pitot tube blockage in an A330. They followed the abnormal checklist for unreliable airspeed which instructed them to switch off all three ADRs.

In the video it is mentioned that with the ADRs switched off that they had to use gravity extension of the landing gear, which caused one of the gear doors to briefly strike the runway. Also it inhibits nose wheel steering requiring a tug to get the plane off the runway.

I can’t figure out why the lack of ADRs would affect these two systems. I know that gear extension would be inhibited above a certain airspeed, but it seems that switching off the ADRs should override this. Why would it require alternate gear extension? And why would nose wheel steering be affected on the ground?

TomMcW
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1 Answers1

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To explain why ADR is needed for the gear operation, refer to this logic diagram:

enter image description here
A330 FCOM; purple highlight added

With no ADR input and with the wheels not on ground, the valve closes. Also note the loop labeled "self maintained", which is a latch that enables keeping the valve opened with the lever up but still below 280 kts (speed input needed).

Now onto the nose wheel steering (NWS): its hydraulics come via the same line as shown above/below, and so it too would be shut off. Also as shown above, the gravity extension shuts off the hydraulic supply regardless of wheels on ground.

enter image description here
A330 FCOM; purple highlight added

Also worth mentioning: NWS also requires the nose wheel doors to be closed, which weren't in this case because of the alternate extension, but that's a secondary reason after the "cut out valve".

Of note: an automatic selector switches to the Blue hydraulic system for braking if there is low Green pressure reaching there. Blue braking remains active in-flight with the lever up (FCOM § 1.32.30).

The safety valve (see this other topic where @OSUZorba explains its uses) is already operated by two systems for redundancy, where only one ADR is needed as shown above. ADR 1+2+3 failure was deemed too remote there isn't even an ECAM message for it, and the QRH is used instead (FCTM § 8.90).

An override in theory can be designed, but it doesn't exist, and I can't speak for the A330's certification/design program.

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    Of course risk of failure of all air data in flight is indeed quite remote. But in this case they were inoperative because of mistake in pre-flight, and the crew could have saved themselves the trouble if they aborted when the airspeed indications were not alive by 80 or 100 knots. – Jan Hudec May 01 '22 at 13:34
  • @JanHudec: True, pilot-error is a factor. I'm surprised by the final report though: "For technical reasons, the main landing gear doors did not retract." The report does not explain or care about the LG/NWS issue. –  May 01 '22 at 13:38
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    I suppose it was considered irrelevant to the cause of the incident (which is mainly about how conspicuous red flags could be missed no less than five times) and not unreasonable. – Jan Hudec May 01 '22 at 14:09
  • @JanHudec: Could be of interest: for a comparative analysis with a Boeing that doesn't have an OFF L/G position like the A330, and if I'm reading the FCOM correctly, in normal operation the B777 L/G freefalls, but once down is locked hydraulically. The B777 FCOM does not mention a speed above which the L/G EXT would be disabled. –  May 01 '22 at 16:42
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    @jan nothing can be "quite remote" when it has a single, shared point of failure. One generally installs all 3 covers or none. I am surprised that GPS or gear axle speed over 50kts + lack of IAS on all 3 sensors doesn't light Master Warning and sound "abort, pitot covers". – Harper - Reinstate Monica May 02 '22 at 00:26
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    @Harper-ReinstateUkraine It's not really a "point of failure". If the pitot covers are still on, you will not get any airspeed indication during the takeoff roll and just reject. See also the Airbus First: Unreliable Airspeed at Takeoff on the issue. Not rejecting was clearly pilot error. – Bianfable May 02 '22 at 06:54
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    @Harper-ReinstateUkraine, I explicitly said risk of failure of all air data in flight. There is no way for the pitot covers to get there in flight (though see TAM A319 near Rio de Janeiro on Jun 17th 2011). The 80 or 100 call should serve as last check to avoid taking off with them not working (here the pilot flying even suggested aborting, but didn't get a response and didn't just do it). – Jan Hudec May 02 '22 at 11:36
  • I'm just saying, there's a long history of "3 of the same thing" all being taken out by the same cause, hence, it provides no more depth than 1 of the same thing. – Harper - Reinstate Monica May 04 '22 at 19:51