B737-800 Multiple pitot tube malfunctions

[KristianNorway]

Dear colleagues,

This is a rather unlikely scenario, but bare with me. What would be the technical implications of all pitot tubes being blocked on a B737-800 at once?

Since both ADIRU's give unreliable information to a large number of secondary systems I would expect much to happen, although I'm not certain about the succession of events or the total amount of systems affected.

I would expect the speed trim system and mach trim system to be u/s since the SMYD receives unreliable data, the EEC to revert to soft alternate mode and naturally all IAS-data to be unreliable. I would also do a flap 30 landing in fear of the pressure in the tubes affecting the flap 40 load relief system. Does the F/D revert to BOV (Bias out of view)?
Can I rely on the GS-indication on the upper left corner of the ND to have some sort of speed indication?

Are there other important technical implications?

Thanks for any input :ok:

[b377]

Pitot-static system - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Pitot-static_system#cite_note-asn-bair301-6)

Try this link

Birgenair B-757 catastrophic hull loss due to blocked pitots following hangar work in 1995 .

[FCS Explorer]

you would probably be flying with shaker and/or clacker all the time.
and not getting past F25 due to load relief. alternate system could help here.
just not sure if you want to fly slower than F15 ref speed in regard to stall. speed taken from GPS.
LNAV/VNAV or ILS approaches, both with manual throttle should still be possbile incl FD.

[hoover1]

don't people fly pitch and power anymore. we all know the approximate power required for a given phase of flight. it would be harder but not that hard to fly an approach with no airspeed indication. i have had to do an approach with no airspeed indicator on a light twin before. not that hard. though i would not want to do this and press my luck.

[FCS Explorer]

:} well, there's still that pitch&power table in the inflight perf section of the QRH !

[KristianNorway]

So then we know we'd have the mach trim system and the speed trim system go u/s, EEC soft alternate modes on both engines and of course no indication on our ASI's.

Depending on the pressure in the pitot and if you're climbing you'd hear the clacker in normal flight.

The pitch and power tables of the performance inflight section in the QRH are good pointers, but quite natural settings if you've flown the 738 manually before.

I'm also unsure if the GS indication in the ND is solely GNSS-based. Can't find it in the books either. Does anyone know?
But I'm sure one can get GS from selecting TK/GS from the ISDU on the aft overhead panel.

Then.. the F/D is an indication of where the FCC wants you to steer. To know this, the FCC receives information from the ADIRU via the FMC. E.g. flying straight and level, then pulling off the thrust will give us a pitch up command from the F/D to maintain altitude. To know this the FCC needs information on atmospheric conditions from the ADC in the ADIRU.
The maintenance manual also states that it will BOV (Bias out of view) if it receives unreliable info from the ADIRU.
But I was told one still has the F/D in this situation.
How can this be?

Looking forward to inputs / corrections :ok:

[FCS Explorer]

for pitch try using the birdie (flight path vector)
for GS try POS page #2. but i think GPS is just giving position, not speed, but IRS's should give GS. also call controler for GS readout. or have someone with working speed indication escort you (other plane/parallel approach).

[HAWK21M]

The B757 accident was Taped on Static ports.Thereafter the AMM lists the sequence of taping the ports using Red streamers.
regds
MEL.

[john_tullamarine]

Minimal exposure to NG but, for interest, on the Classics we used to have endorsement folk be competent with everything failed during T/O that was able to be failed in the way of speed and height information. Requirement was to get back via a raw data, pilot interpreted, extended circuit and ILS to Cat 1 minima or less.

Exercise was done with reference to body angle/thrust setting and rarely took more than a couple of runs for the individual to become comfortable and competent.

[boeing320]

The ground speed is calculated by the ADIRU directly from the GNSS and IRS data and should therefore be correct. You will have inaccurate wind data because the calculated TAS will be incorrect.

The most unsettling thing would obviously be the confusing or misleading IAS and Mach indications and possible stick shaker or clacker due to the incorrectly sensed speed. The high and low speed margins and limits on the IAS tape should still be at the correct speeds, but obviously you don’t know what speed you're actually doing, so not much help. Mach trim / speed trim would operate incorrectly. Flap load relief would operate incorrectly.

How all this actually looked on the pilots PFD would depend on the scenario ie at what stage in the flight the pitot tubes became blocked and whether the aircraft climbed or descended thereafter.

I don’t think the EECs will switch to soft alternate when the tubes are blocked (not immediately anyway, again depends on the scenario).

[David Horn]

Is the stick shaker on a 737 activated by low IAS, or AoA vane, or both?

[boeing320]

Stick shaker is commanded by the SYMD computers, at a given AOA (measured by the alpha vanes) and depending on the wing configuration. No IAS input, as you'd expect. It was late and i'd had a few when i wrote that :ok:

[dynamite dean]

On a biz jet a couple of year ago take off down the runway waiting for the airspeed both call and er come on then....zzzz Abort abort No airspeed data.
Eventually got back to Apron had look and someone we think had stuffed crushed gravel into the tubes!