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Pitot Tube Problems Receiving Ongoing Scrutiny: AvWeek

Discussion in 'Trip Experiences' started by Lord Leighton, Jun 22, 2016.

  1. Lord Leighton

    Lord Leighton Hangar Gold Member I

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    [​IMG]
    Pitot Tube Problems Receiving Ongoing Scrutiny
    Jun 17, 2016 Sean Broderick and John Croft | Inside MRO
    Pitot Problems

    Pitot probe vulnerability is again receiving scrutiny from regulators following detailed reports on two inflight emergencies traced back to the systems.

    The first, involving a United Airlines Boeing 757 descending into Dublin in October 2013, was chronicled in a recent issue of this magazine (AW&ST May 23, p. 32). The report by Irish investigators cited two probable causes: A temporary blockage of the right main pitot tube due to icing, leading to an inaccurate low-airspeed indication on the first officer’s display and the crew’s non-standard response to the low-airspeed reading. The Irish Air Accident Investigation Unit’s report included eight recommendations, including for the FAA to “study whether a safety deficiency exists in pitot probe icing protection” for aircraft certified before January 2015, when enhanced certification standards went into effect. Those changes were triggered in part by the investigation into the June 2009 Air France Flight 447 accident, which determined that inconsistent airspeed readings between the captain’s and first officer’s displays started the chain of events that led to an aerodynamic stall.

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    Leaving pitot tubes uncovered for even a few hours can invite trouble. Credit: ATSB

    A month after the United incident, industrious mud-dauber wasps took less than 3 hr. to build a nest in the pilot’s-side pitot probe of an Etihad Airways Airbus A330 on the ground at Brisbane Airport in Australia, triggering a series of troubling events, an Australian investigation found. The undetected blockage of mud resulted in an aborted takeoff that was followed by an inconclusive troubleshooting effort by maintenance technicians and a second takeoff for Singapore. That departure was quickly followed by a Mayday call by the pilots, who promptly returned to Brisbane. Several organizations affected by the incident instituted changes based on the investigation, an Australian Transport Safety Bureau’s (ATSB) report explained. The airport instituted multiple operational changes, Airbus modified its maintenance troubleshooting manual, and Etihad began requiring ground crews to install pitot probe covers at Brisbane “irrespective of ground time.”

    Investigators determined that the nest blocked the captain’s pitot tube, resulting in a red “speed flag” display on the avionics as the aircraft accelerated through 50 kt. on the first takeoff attempt. Per standard operating procedures (SOP), the captain rejected the takeoff. The A330 has three open-face pitot tubes—a captain’s probe, first officer’s probe and standby probe—on the underside of the fuselage near the nose, devices that measure ram air pressure that is converted to airspeed readings by the avionics.

    Maintenance technicians relied on two procedures in the A330 troubleshooting manual (TSM), neither of which identified the pitot probe as a possible root cause for the airspeed indication problem. The ATSB noted that Airbus had sent out a service letter to operators prior to the incident, linking airspeed discrepancies to potential pitot probe problems. The airframer in October 2014 updated the TSM to include the additional information.

    The A330 was cleared for departure after a few minor avionics configuration changes, but the captain’s airspeed indicator again failed during the takeoff run, this time at a speed where SOPs called for continuing the takeoff. The ATSB questioned the captain’s recollection that the airspeed failed after “V1” (151 kt.), the speed at which crews are advised to continue the takeoff, noting that the flight data recorder information showed that the failure flag should have appeared after reaching 50 kt.

    Once airborne, the sensor issues caused the A330’s fly-by-wire flight control logic to revert to alternate law and various slat and flap warnings occurred. The pilots declared an emergency and landed at Brisbane at an aircraft weight of approximately 200 metric tons, 18 heavier than the A330s 182-metric-ton maximum landing weight.
     
  2. Irish MD-11

    Irish MD-11 Hangar Bronze Member V

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    Just what is the diameter of the "entrance" to a pitot tube, does it depend upon the aircraft's size?
    Why not redesign the entrance slighty? That would need just to include a sort of a fine sieve affixed to the entrance of the pitot tube to block any foreign objects(that have no business in there, such as mud-dauber wasps,etc.);)! Just like what Mom would use to strain the water from a can of peas, "without spilling the beans":D:D:D

    Any ideas out there folks?

    Irish MD-11
     
  3. Lord Leighton

    Lord Leighton Hangar Gold Member I

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    Any object no matter how fine, small, or invisible would render a pitot tube useless. If they put some type of reverse blow out mechanism to be applied before any flight in question to clear any possible debris along with a pitot tube obstruction detection device readable from the flight deck, might be a viable option. If my memory serves me right, the entrance to a pitot tube is any wheres around 2-4 mm depending on plane size and speed capabilities?
     
    Last edited: Jun 23, 2016
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  4. Exuma Guy

    Exuma Guy Hangar Silver Member V

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    You are correct, and a blow-out would be good. Design it so that the valve shuts off pressure to the sensor to prevent damage before it pushes pressure into the line.
    This would also cure some problems like rain water collecting in the static ports which used to happen regularly on my Aztecs.
     
  5. Lord Leighton

    Lord Leighton Hangar Gold Member I

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    I know pitot tubes are supposed to be heated but I'm not sure if that is the case in ALL A/C. Rain inside them turning to ice has been a problem in the past when the heaters have failed causing the auto-pilot to screw up and planes entering high speed flat stalls in many instances. Then the dreaded pilot's words "What's the plane doing?"
     
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  6. Shawn White

    Shawn White Hangar Silver Member I

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    Hehe, the blowout idea actually sounds pretty great. On smaller aircraft it's easy to check the pitot tube during the preflight, but on anything larger it's inconvenient. Pitot heat for some planes is anti-icing but not de-icing, and it also doesn't guarantee accurate readings in all icing conditions. Although I'd assume for a transport category this wouldn't be an issue??
     
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  7. Exuma Guy

    Exuma Guy Hangar Silver Member V

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    AF447's chain of events started when the pitot tubes were blocked by ice because the heaters weren't effective.
     
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  8. Irish MD-11

    Irish MD-11 Hangar Bronze Member V

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    Thanks for your reactions guys! Yes, a blow-out system sounds like a really cool idea:)! Can't be a "mountain to be moved" job;)!

    Irish
     
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  9. Lord Leighton

    Lord Leighton Hangar Gold Member I

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    Yeah, I'm wondering why this concept hasn't been applied already? I'm certainly no rocket scientist (yet), and I'm certain it wouldn't take a rocket scientist to design a relatively low cost system to incorporate. Oh I forgot, in the aviation biz, a simple washer made by an OEM probably costs $100.00, a bogus Chinese washer with fake papers? 10 cents if that, to make.
     
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  10. MPilot

    MPilot Hangar Associate Member II

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    On preflight outside check of airliners there is hardly a chance to detect pitot tube blockage except with the use of a pretty heigh ladder, which is not done in practise, but it should be absolutely! Ironically the pitot check of a C172 is carried out more carefully. Anyway, considering the described case of wasps building a nest or the lot of incidents and accidents caused by icing, industry should think about if airspeed gauge by pitot, a device referring to Henri Pitot's publication in 1732, is still appropriate to feed high tech avionics in the auto flight mode. Even though a well trained pilot will be able to steer on attitude and power indication without speed monitoring, its time to adopt new air speed measurement technics. There are lots in the drawers of universities.
     

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