View Full Version : FMC non-acceptance of ATM with high OAT
JammedStab
20th Feb 2013, 01:14
Has anyone had this situation happen with high OAT and no APU running where the FMC will not accept the assumed temperature.
Something to do with bleed air to the TAT probe.
rudderrudderrat
20th Feb 2013, 05:49
Hi JammedStab,
It'm still amazed that an OAT probe can indicate correctly, even when the heater is on. This link provides a clue to how the clever designers can do it:
patents/US6370450? (http://www.google.com/patents/US6370450?printsec=drawing#v=onepage&q&f=false)
On this particular type, in figure 3, APU speed is considered.
Was is possible that the measured OAT was above your Assumed Temperature?
If so, the FMC will reject it.
5420N
20th Feb 2013, 20:41
Yep, I've had that before.
It's because the TAT probe is not aspirated by bleed air and therefore not reading the correct temperature.
Guidance in our manuals was to wait until bleed air available, i.e. engine start and then entering the assumed temperature.
Cheerio
mutley320
20th Feb 2013, 22:25
I believe the FADEC has its own temperature sensor and if the OAT (sensed by the FADEC) is higher than the FLEX temp, the ECAM will just show TOGA until the FADEC sensor "cools" down to actual temp.
JammedStab
21st Feb 2013, 01:01
Yep, I've had that before.
It's because the TAT probe is not aspirated by bleed air and therefore not reading the correct temperature.
Guidance in our manuals was to wait until bleed air available, i.e. engine start and then entering the assumed temperature.
Cheerio
Thanks,
That was the answer I had before. Now for the key question. What does aspirated by bleed air mean?
What does aspirated by bleed air mean?
Air from the bleed system is fed into the TAT probe(s) to assist outside air to flow over the TAT probe's sensor. This is known as aspiration.
The air from the bleed system, whilst originally hot, has to travel down a maze of pipes (often narrow) and gets to the TAT probe at a relatively low temperature (thus not adding to the temperature of the TAT probe).
Without this aspiration, direct sunlight as well as electric TAT anti-ice heat can increase the temperature reading by a significant amount at low airspeeds.
I had a problem with a 747-400 Boeing Converted Freighter (BCF) a while back, where, during conversion, the plumbing from the bleed ducts wasn't hooked up and caused an ongoing problem with high TAT readings.
latetonite
21st Feb 2013, 07:50
Are you able to enter the correct outside temperature in your FMC, if you have aspirated pitot`s?
JammedStab
23rd Feb 2013, 01:00
Air from the bleed system is fed into the TAT probe(s) to assist outside air to flow over the TAT probe's sensor. This is known as aspiration.
The air from the bleed system, whilst originally hot, has to travel down a maze of pipes (often narrow) and gets to the TAT probe at a relatively low temperature (thus not adding to the temperature of the TAT probe).
Without this aspiration, direct sunlight as well as electric TAT anti-ice heat can increase the temperature reading by a significant amount at low airspeeds.
O.K.......Is there some sort of venturi principle here for operation.
I don't suppose you have a diagram of operation or quote from the maintenance manual.
O.K.......Is there some sort of venturi principle here for operation.
Seems to be the case. The AMM just says the bleed air "causes" air to be drawn through the TAT probe and over the sensors. Our training notes just say that the bleed air causes a negative pressure which pulls air across the sensing elements.
(EDIT: I have diagrams, but they are not for distribution)
P.S. By the way, on Boeing 737s with aspirated probes, the TAT probe is low enough for you to reach up and feel the air exiting the back of the probe. Don't worry, it won't burn you ;)