Boeing 737 TAT indications
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Boeing 737 TAT indications
Have noticed (on the B737- classic as well as the NGs) that after engine start, the TAT reading is showing +99 degrees celcius, even with an aspirated TAT probe. Anyone with clues as to why?
Its not an TAT probe problem because on the bay the TAT has been showing pretty much the accurate temperature.
Its not an TAT probe problem because on the bay the TAT has been showing pretty much the accurate temperature.
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FWW - the TAT probe is aspirated by the a/c system, which is off for engine start. Therefore on any aspirated 737 system the reading is totally unreliable until packs have been running for a while and on the 200, until airborne.
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The probe is also electrically heated, and the heat is switched on just after engine start. On stand, the heat is switched off, the number will be representative of the OAT. When the heat is applied and combined with low forward speed, the heat cannot be dissipated and builds up in the probe which then gives a reading of up to max +99C. Once airborne it will quickly return to normal.
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Minuteman,
I don't know of any aircraft where the heater is powered on the ground unless someone is playing with the ground sensing system, NG or classic, regardless of Pitot Heat Switch position.
The thing gets very quickly to 750 degrees F. It's a safety issue.
I don't know of any aircraft where the heater is powered on the ground unless someone is playing with the ground sensing system, NG or classic, regardless of Pitot Heat Switch position.
The thing gets very quickly to 750 degrees F. It's a safety issue.
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????
As per Avionikers statement "From my understanding it is only supplied low voltage (26)on the ground even when selected on and then high (115)when the air/groung logic takes over...But I could be wrong !!!
As per Avionikers statement "From my understanding it is only supplied low voltage (26)on the ground even when selected on and then high (115)when the air/groung logic takes over...But I could be wrong !!!
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It is true that the normal heaters are heated with reduced voltage on the ground and full 115v in the air. The exception is....... you guessed it. The TAT probe.
From the MM.
'The system provides dual level pitot-static probes and temperature probe
heating through air/ground logic circuits in the E11 landing gear logic
shelf. When on the ground, the system provides reduced voltage to the
pitot static probes and removal of voltage from the temperature probe,
thus prolonging probe element life.'
This doesn't answer the original question though!
From the MM.
'The system provides dual level pitot-static probes and temperature probe
heating through air/ground logic circuits in the E11 landing gear logic
shelf. When on the ground, the system provides reduced voltage to the
pitot static probes and removal of voltage from the temperature probe,
thus prolonging probe element life.'
This doesn't answer the original question though!
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boac, minuteman- so thats two possibilities- the packs not running for long enough or the heat supplied to anti-ice the TAT probe not dissipating and causing heating upto 99deg.
TAT= SAT + Ram Rise, Why would one want to heat the TAT probe without a check, that it does not accurately indicate the OAT. My car indicates the OAT fairly accurately... and it does'nt cost anywhere close to 60 Million.....
Another thought, - the Take off page on the FMC get a OAT input from the TAT probe, which need not be hard selected or overwritten with another valu- based on this the Toff N1 is displayed- this reading wouldnt be accurate if the TAT went to 99 degrees or would it ???????????
TAT= SAT + Ram Rise, Why would one want to heat the TAT probe without a check, that it does not accurately indicate the OAT. My car indicates the OAT fairly accurately... and it does'nt cost anywhere close to 60 Million.....
Another thought, - the Take off page on the FMC get a OAT input from the TAT probe, which need not be hard selected or overwritten with another valu- based on this the Toff N1 is displayed- this reading wouldnt be accurate if the TAT went to 99 degrees or would it ???????????
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FWW -
Q1: I guess it is better to have it de-iced than not?
Q2: I ALWAYS enter the OAT on the cdu regardless of type of 737 on the basis that if the TAT system failed and ran away 'high' I would always have the right MAX N1 if I needed it. Remember that 99% of the time you are running at a 'false' OAT on reduced power, so failure is not a problem.......unless you are full power or engine out.
Q1: I guess it is better to have it de-iced than not?
Q2: I ALWAYS enter the OAT on the cdu regardless of type of 737 on the basis that if the TAT system failed and ran away 'high' I would always have the right MAX N1 if I needed it. Remember that 99% of the time you are running at a 'false' OAT on reduced power, so failure is not a problem.......unless you are full power or engine out.
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FWW
Q1: boac is correct. And seeing as within 2-3 minutes after departure, the reading is back to normal, you are in a good position to make an assessment of the reading, and decide if it appears incorrect.
Q2: The Take-Off page, at least on my machine, has an OAT input which I manually key in. But I take the value from the ATIS (or equivalent) rather than the TAT indication. So the machine bases the TO N1 on my input regardless of the TAT indication. I can't speak for the NG FMC.
Q1: boac is correct. And seeing as within 2-3 minutes after departure, the reading is back to normal, you are in a good position to make an assessment of the reading, and decide if it appears incorrect.
Q2: The Take-Off page, at least on my machine, has an OAT input which I manually key in. But I take the value from the ATIS (or equivalent) rather than the TAT indication. So the machine bases the TO N1 on my input regardless of the TAT indication. I can't speak for the NG FMC.
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From my reading of the reply that Mono provided which quoted the manual, there is no voltage to the TAT probe when on the ground;
".........and removal of voltage from the temperature probe,
thus prolonging probe element life"
Regards,
DFC
".........and removal of voltage from the temperature probe,
thus prolonging probe element life"
Regards,
DFC
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Guys, on the 737NG with the asprirated TAT probe option:
Is the logic for pitot heat through air/gnd?
Does it get it's air from the packs or from the pneumatic system?
I only ask because on our 757s which have the asiprated probe, it gets air from the pneumatic system. And on our 737NGs, pitot heat has no interface with air/gnd. It is manual only.
Is the logic for pitot heat through air/gnd?
Does it get it's air from the packs or from the pneumatic system?
I only ask because on our 757s which have the asiprated probe, it gets air from the pneumatic system. And on our 737NGs, pitot heat has no interface with air/gnd. It is manual only.
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From 737NG SDS for aspirated probes: 30-30-00-035
ICE AND RAIN PROTECTION - PITOT AND STATIC - TAT PROBE -
FUNCTIONAL DESCRIPTION
Functional Description The total air temperature (TAT) probe anti-icing system uses electric power and resistance-type heating elements.
The system uses 115v ac and 28v dc power. The probe heating element uses 115v ac power. The current detection circuit uses 28v dc power.
On the ground, put the control switch to the ON position. This energizes relay K1 and stops 115v ac power through the current detection circuit to the probe heater. The amber TEMP PROBE light does not come on.
Push the TEST switch (with control switch in ON position) to deenergize relay K1. This lets 115v ac power through the current detection circuit to the probe heater. If the probe heater does not use current, the circuit causes the amber TEMP PROBE light to come on.
In flight, relay K1 remains deenergized.
~~~~~~~~~~~~~~~~~~~~~~~~
The Low voltage is for excitation and signal generation. The high voltage is for heat.
In an aspirated probe the bleed air is to ensure a continuous flow of air through the probe and across the sensor.
~~~~~~~~~~~~~~~~~~~~~~~~
Then there's SDS: 34-21-00-023
ADIRS - TOTAL AIR TEMPERATURE PROBE
General
The total air temperature (TAT) probe measures the air temperature outside the airplane. An opening in the front of the sensor lets air flow through and around the sensing elements and exit through ports in the rear of the probe.
Sensing Elements
There are two sense elements in the TAT probe. Each sense element is a resistive element. The value of resistance changes when the temperature of the air flow across the element changes.
Bleed Air
The TAT probe gets bleed air from the APU duct in the keel beam. See the bleed air chapter for more information. (CHAPTER 36) Bleed air into the probe makes a negative pressure inside the probe. The negative pressure pulls outside air across the sensing elements. This permits accurate temperature measurement when the airplane is on the ground or moving at low speed.
~~~~~~~~~~~~~~~~~~~~
I can't find a thing in any of the manuals avaliable to me that would satisfactorily explain the reading that generated the original question.....gotta love this business
ICE AND RAIN PROTECTION - PITOT AND STATIC - TAT PROBE -
FUNCTIONAL DESCRIPTION
Functional Description The total air temperature (TAT) probe anti-icing system uses electric power and resistance-type heating elements.
The system uses 115v ac and 28v dc power. The probe heating element uses 115v ac power. The current detection circuit uses 28v dc power.
On the ground, put the control switch to the ON position. This energizes relay K1 and stops 115v ac power through the current detection circuit to the probe heater. The amber TEMP PROBE light does not come on.
Push the TEST switch (with control switch in ON position) to deenergize relay K1. This lets 115v ac power through the current detection circuit to the probe heater. If the probe heater does not use current, the circuit causes the amber TEMP PROBE light to come on.
In flight, relay K1 remains deenergized.
~~~~~~~~~~~~~~~~~~~~~~~~
The Low voltage is for excitation and signal generation. The high voltage is for heat.
In an aspirated probe the bleed air is to ensure a continuous flow of air through the probe and across the sensor.
~~~~~~~~~~~~~~~~~~~~~~~~
Then there's SDS: 34-21-00-023
ADIRS - TOTAL AIR TEMPERATURE PROBE
General
The total air temperature (TAT) probe measures the air temperature outside the airplane. An opening in the front of the sensor lets air flow through and around the sensing elements and exit through ports in the rear of the probe.
Sensing Elements
There are two sense elements in the TAT probe. Each sense element is a resistive element. The value of resistance changes when the temperature of the air flow across the element changes.
Bleed Air
The TAT probe gets bleed air from the APU duct in the keel beam. See the bleed air chapter for more information. (CHAPTER 36) Bleed air into the probe makes a negative pressure inside the probe. The negative pressure pulls outside air across the sensing elements. This permits accurate temperature measurement when the airplane is on the ground or moving at low speed.
~~~~~~~~~~~~~~~~~~~~
I can't find a thing in any of the manuals avaliable to me that would satisfactorily explain the reading that generated the original question.....gotta love this business
