NSEU
19th Mar 2009, 11:47
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).
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).