Excessively High TAT on 744 Freighter

[NSEU]

Our maintenance crew was called to a certain airline's 744 freighter because of an excessively high TAT reading on the ground (38C versus 27C on ATIS). The flight crew was anxious to leave, and wanted us to apply an MEL. However, the MEL/DDG only allowed a release if the SAT was ok (highly unlikely, as the SAT is computed from the TAT in the Air Data Computers).

Our theory was a little rusty, but we thought we'd take a look at the Left TAT probe first (and ordered a new probe). The data path from the TAT probes to the Upper EICAS is not explained well in the manuals, but in hindsight, we could have pulled both FMC CB's and the Left ADC CB, forcing data to come from the Right TAT probe, to allow us to make a comparison.

However, when we reached the TAT probe with our mobile stand, we discovered that there was no aspirating air coming from the probe.
Looking at the parts manual, both the Left and Right TAT probes and, incidentally, the Rain Repellent system use the same source of air (left pneumatic bleed duct), so it was very likely that the Right TAT probe would also be giving a false reading.

The flight crew were talking about some kind of dispensation (using manually inputted temperatures into the FMC), but the paper trail would be never ending, so it was decided to ground the aircraft to allow for troubleshooting.

This was a rather daunting task as the plumbing for this system would be something like 150 feet long (and most of it is hidden behind panels with lots of screws in them (freighters have very few easily removable panels.. I assume for additional fire protection).

Many, many hours later, the problem was found ... a disconnected and capped connection at the bleed duct (behind the wing-to-body fairing panels) ... An obvious maintenance error. We discovered later than this aircraft had a 6 month history of this problem (but I assume the problem only surfaces when strong direct sunlight is heating up the probes).

Even so, a lot of us are shaking our heads at the time it took for this particular airline to actually allow someone enough time/resources into fixing this problem. Has this aircraft been unsafe for 6 months? Surely a high TAT reading on the initial ground takeoff roll would tell the engines to throttle back? (or do flight crew procedures mandate a manual entry, irrespective of TAT reading?).

If both TAT probes are showing the same temperature, but the engine sensors are showing something different, how do the EEC's (CF6) know which source to select? (my manuals are not clear on this).

Thanks for any insight.

Cheers.
NSEU.

P.S. By the way, the primary source of the Upper EICAS TAT readout is the Master FMC, with a backup readout from the EIU's (directly from the ADC's). However, we are not sure if the FMC's share data (and would use data from any valid TAT probe, irrespective of the FMC Master switch position).

[Willit Run]

My question is; How is the TAT calculated without an SAT? That is the base line temp from which the calc's are made.

[NSEU]

Looking at the schematics of the Air Data Computer internals, Boeing use the temperature sensor data from the TAT probe and combine it with Mach to produce a TAT value. Microprocessors do all the number crunching.

This TAT information is then fed into another microprocessor, along with mach (again!) to produce SAT.

Weird science

[Cornish Jack]

NSEU
A little caution with respect to the "Master FMC". Been out of the loop for a while, but my addled memory tells me that the "Master" function is fixed and NOT dependent on the FMC Master position. That switch only dictates which FMC will source displays rather than information. Afraid my manuals are many miles away in storage but it might be worth your while having a grub around in the 'heavy' books.

[NSEU]

Hmmm... not really what I wanted to hear, CJ :P

I found comfort thinking that the 744 was one of those rare aircraft on which you did know which FMC was the Master.

The Navigation displays are independent (wrt FMC map data), but in matters of thrust control, I believe there are interlocks (e.g. you can't arm the autothrottle if the FMC L/R switch disagrees with Master FMC relay).

Cheers.
NSEU

[Intruder]

Cornish Jack is NOT correct.

In the 744 the "Master" FMC is the one selected by the switch in the upper center instrument panel -- usually the Left. While the FMCs update each other, only the Master provides actual guidance commands to the other systems. If the Left FMC fails, the switch has to be manually moved to Right to regain Autoflight (VNAV, LNAV, Autothrottles).

Independent of that switch, the PFDs and NDs are fed by the FMC selected on the respective Capt/FO Source Select switches -- usually Left for Capt and Right for FO.

[barit1]

My question is; How is the TAT calculated without an SAT? That is the base line temp from which the calc's are made.

In flight, it's impossible to measure SAT directly - wherever you try to put the probe, it's affected by local Mach.

So the best you can do is measure TAT, then "back out" the Mach correction to compute SAT.

[Short_Circuit]

If both TAT probes are showing the same temperature, but the engine sensors are showing something different, how do the EEC's (CF6) know which source to select? (my manuals are not clear on this).

From memory, the EEC's have Prim & Sec computation sections that only use data from local sensors to do all its own calculations, only if there is a internal voting error does the EEC look at A/C data, Prim at L ADC & Sec at R ADC. If there is an error with external data, the EEC will use internal generated assumed data using the local sensors available to it. Not too sure off hand if this is RR or GE or both (similar).

[fflyingdoguk]

I had a problem in Nbo with a higher than advertised TAT ,after carefull trouble shooting i found that the(still running) tug that had been sat under the aircraft for a couple hours was directly under the probe and its radiators were very helpfully disapating the heat right under the probe ,problem solved.

[Spanner Turner]

From memory, the EEC's have Prim & Sec computation sections that only use data from local sensors to do all its own calculations, only if there is a internal voting error does the EEC look at A/C data, Prim at L ADC & Sec at R ADC. If there is an error with external data, the EEC will use internal generated assumed data using the local sensors available to it. Not too sure off hand if this is RR or GE or both (similar).

SC, you're memory is good but just a little off. All of the 'electronic' engines i've worked on GE CF6, RB211-524 and P & W 7R4 and PW4000 all use the ADC inputs for TAT, MACH and P0 (ambient press).

Yes, they all have their own built-in engine sensors for these parameters but only default to these if the ADC input is lost and/or is outside of the EEC's internal error checking limits (i.e the EEC takes the ADC input for say P0 and checks it against it's own P0 sensor - if the two figures are within an acceptable tolerance the ADC data is the one used for calculations by the EEC)

As was explained to me by a very good instructor, more money is spent on the building/testing/certification of the ADC's and this data is good enough to operate the auto-pilot, displays for pilots, ATC encoder etc, so it's the best info for the engines to use too.

Of course the engine EEC's need other parameters NOT measured by the ADC's so in those cases use attached engine sensors.



P.S. will dig into the books and see if I can find some more info.

.

[NSEU]

After further reading (if I understand this correctly), on CF6's, a comparison is made between the two T12 (engine inlet temp) sensors and if they disagree with the airplane sensor, then the EEC's switch to a soft reversionary mode.

I guess, there being 2 engine sensors, it would be easy to see if the aircraft sensors are faulty.

My notes say that a soft reversionary mode may cause thrust asymmetry, so I guess it's not what you want on takeoff. There are associated EICAS messages with soft reversion, so I guess the crew wouldn't even get as far as the runway with an airplane TAT so different from the engine sensors.

My schematics show information from two ADC's going to each FMC (via the Instrument Source Select switches) complicating the matter further. The only way I can see of forcing the FMC to use the Right TAT probe is to select both ADC Instrument Source Switches to Centre. The ISSS's offer onside or Centre ADC info to the FMC, the Centre ADC being fed by the second element in the Right TAT probe.

BTW, Cornish Jack is right 99.9% of the time, so if he's wrong this time, I'll forgive him