TAT probe

[Jonty]

Does anyone have any info on what flight instruments may be affected by the TAT probe in a B757 being iced up?

[QAVION]

I guess modern Boeing's are pretty much the same re TAT.

On the 747-400, TAT is used to calculate SAT and TAS in the Air Data Computers.

You don't need these things on a 757 do you? :)

[411A]

If the TAT was inaccurate, then all performance data that relied on temperature in the FMS would also be inaccurate.

[Jonty]

Cheers Qavion,
The story behind this is that we lost TAT probe heat on take-off and our QRH just said "flight in icing conditions may result in some erroneous flight instrument indications"
We also lost Autothrottle, and I wonderd how the two relate. http://www.pprune.org/ubb/NonCGI/confused.gif

So we DO need it!! :)

411A,
Would this also affect the autothrottles evaluation of thrust required due to the OAT not being available?

Our manuals are cr@p!!! :mad:




[This message has been edited by Jonty (edited 17 February 2001).]

[QAVION]

"Our manuals are cr@p!!!"

I know the feeling!
According to my 747-400 notes, when the aircaft is operating in VNAV Speed mode, the A/T computer (which is contained within the Flight Management Computer on a 747-400), calculates a desired TAS value and compares this with the current aircraft "airspeed" to generate a difference value. This difference value drives the throttle levers. I know TAS is fed to the FMC from the DADC's (probably along with lots of other things), but I can't state unequivically that the "current aircraft airspeed" in my text is _TAS_, but this would make sense. So I can see how TAS might affect A/T engagement in at least one A/T mode.

Does your 757 have digital engine control (FADEC)? If the EEC's are using TAT data provided by the TAT probe(s) mounted on the airplane nose, this may also be a factor in A/T disengagement.

Anyway....Hope this helps.
Cheers.
Q.

[Jonty]

Thanks Q,

From talking to other people it appears that to produce an accurate EPR indication the TAT has to be taken into account. So thats one of the instruments affected.

[ROGER Co-J]

A related question :)oes the TAT info also affect the IRS operation, seeing as TAS is part of the " Wind Triangle" needed to work out true wind together with track, groundspeed and heading?

[QAVION]

"Does the TAT info also affect the IRS operation,..."

Yup.
TAT is used to calculate TAS in the ADC's. TAS is then fed into the IRS's.... and winds will not be able to be calculated without this.

Cheers.
Q.

[Feret]

I once had a "TAT Probe" EICAS Msg and light appear during CZ in a B762. The TMC failed as a result, so the A/T disconnected. VNAV also dropped out but could be reselected after the A/T was switched off on the MCP.
Cheers,
Feret.

[Golden Rivet]

For 757 ,

when the TAT heater is inoperative on single TAT aircraft, the TMC considers temperature from the TAT to be invalid and disconnects the autothrottle. On dual TAT aircraft, airplane systems use the unaffected TAT signal when reconfigured to use Center Air Data.

With an inoperative Thrust Management Computer, the Autothrottle System will be inoperative and
the following displays on EICAS will be affected:
1. EGT (and N1, N2, N3 for RR engines) amber indications which are normally inhibited during takeoff or go-around will not be inhibited,
2. Reference EPR will not be available,
3. Assumed temperature (if used) will not be available,
4. Thrust reference mode (TO, CLB, etc.) will not be available.
5. Airplanes that do not have speed tape: the SPD flag may remain in view on the EADI.

With reference FADEC engines, if TAT probe heat fails EEC wiil receive an TAT invalid signal and default to using its own temp sensor ( aircraft data being preference .)


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Fools rush in where Angels fear to tread

[Jonty]

Feret, that was the exact same problem we had.

Golden Rivet,
where were you when we needed you?!!! And why couldnt the QRH say somthing along those lines.

Just one question? our aircraft have two TAT sensors but within the same housing (on the right side of the aircraft, next to two doors that nobody seems to know about). If this housing lost its heat, I would assume that both TAT sensors would be U/S and selecting centre air data would have no effect. Is this true? also if you can shed any light on what the two doors do (right hand side, fwd of the fwd cargo door) it would be appreciated

Cheers!! :) :) :)

[Golden Rivet]

Jonty

Which aircraft are we talking about ? 757 I assume.

The two mysterious doors on the r/h side of the nose are the negative pressure relief valves which prevent cabin pressure from becoming less than ambient in the event of an emergency decent ( perish the thought ! )

The negative relief doors are spring loaded closed and open on the negative cabin pressure of - 0.75 psi thereby preventing the airframe squishing itself.

As for the dual TAT ? I think I'll have to do a bit of research on that one !

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Fools rush in where Angels fear to tread

[This message has been edited by Golden Rivet (edited 03 March 2001).]

[Golden Rivet]

Double TAT probe( L/H & R/H of Nose )only to be found on 757-300 which is fitted with 3 ADIRU's ( combination of ADC & IRU in same box )instead of 2 ADC's and 3 IRU's.

Bog standard 757-200 TAT probe fitted with twin sensing elements and one heating element( R/H side of nose fwd of those mysterious doors ! )Loosing the heater element will cause both TAT sensing element signals to become invalid to certain systems.

If you think that loosing the TAT heater causes you problems, you should see what happens on the A320 if you lose the captains clock !!!




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Fools rush in where Angels fear to tread

[Jonty]

Thanks for the info GR, just one more question regarding those doors, if you dont mind.

Aren't the negative pressure relief doors on the left side of the aircraft aft of the wing fairing? Or do we have two sets?

[Golden Rivet]

Surely your winding me up now !

What do they teach pilots these days about the airplanes they are flying ? Please tell me your not a Captain !!

The two pressure relief doors aft of the left hand wing root fairing are POSITIVE pressure relief doors as opposed to the NEGATIVE relief doors at the front.

Positive pressure relief valves prevent differential pressure from exceeding stuctural limits in the event of a pressurisation system malfunction. The valves have a primary metering section limiting positive pressure to 8.95 psi diff and a secondary metering section as a back-up which operates at 9.42 psi diff.

These valves should be inspected each preflight and if opened a flag is visible.
(not uncommon so keep your eyes open when doing your walk-around )

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Fools rush in where Angels fear to tread

[Jonty]

I can tell you what they teach us on a conversion course, FU@K ALL!!!!

Our manuals dont mention the doors at all, It talks about the pressure relief system stating the pressures you have said, but nothing about doors.

It also says nothing about a dry bay in the wings or the drain holes inboard of the engines. :mad:

What they give us is the MEL a mobile phone and the number of maintroll to phone if we see somthing we dont like.

I think the attitude is the enginers will tell you if its serviceable so you (the pilots) dont need to know about it, God bless the JAA :mad:

Thanks for the info GR.

PS asked meny people about the doors and no one knew about them (Pilots that is)