Absolute minimum temperature?
 

Is there a Boeing limit of absolute minimum OAT? (Not minimum fuel temp)
The lowest I personally observed was -74*C at FL450 over the Baltic, but was told by a BA F/E that he had seen -85*C over Siberia.
Is there a limit and why? Thanks.

Most aircraft have an environmental envelope published in their AFM, for the B737NG the lower limit is -73.5C from FL360 and above.

Pretty sure somebody else can elaborate on why exactly it is needed.

For many things, they can only certify what they test to.

I thought this thread was a discussion on thermodynamics. I’ll get my coat.

Good idea, it might get a bit cold out there🤣

Not quite - you're allowed to extrapolate to some extent, but generally allowed when there is some previous data that validates the extrapolation methodology.

Another issue is many of the data tables only extend over a specific temperature and altitude range - e.g. engine ratings. I recall an issue perhaps six or seven winters ago when a big cold front hit part of Europe - it was bitter cold, but more importantly the pressure altitude dropped well below -1000 ft. The takeoff performance data for some types didn't go below -1,000 ft., resulting in several aircraft being grounded for several days.

Most tends to be a fuel gel point, read about a DC10 over Russia years back that was reaching min tank temp. ATC wouldn't allow any deviation lower/faster etc....... the F/O suggested wing deice, the temp slowly rose due to hot bleed air in the leding edge. Several years back air canada was stuck in yzf for 2 weeks because the temps didn't rise above -48 during that time, what jack@ss makes a skidoo that doesn't run in the winter. Oh right some PQ numb nuts

Been plenty of incidents over the years with fuel gelling. BA38 comes to mind. An aircraft which I was looking after, some years ago, nearly came to grief on the way to NZ - fortunately the PIC recognised the symptoms from a conference paper several years prior, descended, and the problem resolved.

BA 38 was caused by water ice crystals in the fuel, not the fuel itself approaching its wax point.

-70C/FL360 over NE Russia is my record.
I’m going with temperature limitations on lubricants used.
Flight controls, gear, flaps, jackscrews etc.
Just a SWAG
:8

Don't recall ever seeing an environmental limit for the 747-400 or the 777.

Stolen shamelessly from the inter-web. Continental's 777-224, dated 08/09/02 (for whatever Christmas sake that means?).

https://cimg1.ibsrv.net/gimg/pprune....7e883732a9.png
The left edge-line is relevant, showing a binding limit at -75° C when above FL315.

On Airbuses the FCOM chart is pretty much the same.

Isn’t it clear? It’s the 8th of the 9th of the 2nd

;)

Flight planning and performance manual...
 

https://cimg3.ibsrv.net/gimg/pprune....fa464b7466.png
theory is, since -65C is the lowest temp shown, we should limit our flight to -65C

AFM, under the performance section. ​​​​​​

70 Mustang Your charts say TAT, mine OAT. Reader beware.

Check Airman It's the spirit of the date, not the format that matters. :-)

For the 737 I could not find the environmental envelope chart in any of the FCOMs I caught online. Somebody above said it's inside the AFM.

The graph I posted is from Continental Airlines 772 FCOM, 2002 edition. Probably heavily tailored. https://curiozitydotnet.files.wordpr...ght-manual.pdf

The 747 FCOM found is ZK-SUI from ILFC and content-wise looks identical with the 737 in the Limitations Chapter. No Environmental Envelope Graph.

TAT -65 is really cold, check the SAT

Or very slow!

https://cimg5.ibsrv.net/gimg/pprune....52f4d2fa95.png