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January 16, 2017 by

B737NG operating temperatures


Question again out of the field of operation: 

An aircraft requested a descent due to low SAT/OAT of – 70°C, is there such a limit on the B737NG?

It looks that we know the answer related to the minimum FUEL temperature in the tank (#1). For takeoff and in flight this is the highest of -43°C, or 3°C above the freezing level of the fuel in the tanks.

Fuel freezing level temperature specifications are:

– JET A1 (JP1) _ freezing point – 47°C
– JET A _ freezing point – 40°C
– JP-5 _ freezing point – 46°C
– JP-8 _ freezing point – 47°C

Any fuel ice preventing additive does not change the minimum fuel tank temperature limit and (just as a guideline), fuel cools depending on fuel quantity and OAT about 3°C/hr.

The actual answer to the question relates to the aircrafts operating OAT’s which can be found on the ENVIRONMENTAL ENVELOPE diagram in the B737NG AFM.

I’ve highlighted the in flight minimum and maximum OAT limits, and also the takeoff and landing OAT limits can be determined to the current PA, and ISA deviations.

Takeoff and landing limits are bounded at sea level and with standard day conditions from:

– Lower OAT limit at – 54°C
– Upper OAT limit to + 54°C until + 39,4°C at 8.400 feet PA. (maximum T/O and LND field elevation)

In flight the lower limit at standard day conditions depends on PA, and varies from:

– Lower OAT limit at 0 ft PA, – 62°C
– Lower OAT limit at FL 410, – 73,2°C

(mind ISA adjustments)

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