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

https://cimg9.ibsrv.net/gimg/pprune....8bd56a4c2f.png

Competitor's airframe. Ugly point at FL330 with -65 degrees,
- Avionics ventilation gets confused and misbehaves, although nobody understands what the skin heat exchanger normal operation is anyway.
- Fuel temperature in the outer, wing-tip tanks is not manageable over extended periods of time (low limit for JET A1 is -45 or so)

It would be clear except for the fact that in the good old US of A you swap the day and the month around compared to the rest of the English speaking world.

https://cimg8.ibsrv.net/gimg/pprune....7535a687cc.jpg
https://cimg9.ibsrv.net/gimg/pprune....3f5a8263fd.jpg

I was told, some years ago, that the min temp was for 'metallurgical' reasons (Airbus)
mcdhu

John T
 

BA 38..jury out..mate operated the first service and wrote to his manager about the intense cold over Siberia..told not to worry.
Apparently the fuel wasn't allowed to settle after refuelling before the water check in Peking.
Crew ignored the planned descent into warmer warmer in the later stages of the flight.
iirc no fuel heat.
On the iron duck we had different min fuel temps depending the type uploaded.
Could have been either ice or wax but what is definate it didn't put BA in good light.

Back in the mid 70s, we were conducting an external load lift of a 105mm howitzer in the Norwegian mountains. The indicated OAT was -44 degrees C. The recirculating snow was an issue but dissipated quickly although much blew into the cabin through the open doors. My lasting memory is of the crewman using a brush to clear the snow out of the door. The joy of operations in such cold temperatures is that the snow does not stick. Engine and airframe entirely happy but not sure about the PUMA HC1 ODM.

I don't recall any military jets falling out of the air at high altitudes. Do you suppose that air friction might be significant in limiting the metallurgical effects

BA38 was conclusively determined to be water ice in the fuel, not the fuel reaching its wax point. The event led to fuel/oil heat exchanger changes required by AD 2010-07-01.

https://lessonslearned.faa.gov/ll_ma...abID=1&LLID=79

I share your confusion over the newly suggested fact. The possibilities are
- above tropopause the temperature gets warmer
- we're discussing ISA -20 conditions so far, which at high flight levels are quite rare (frequency of exposure) yet encountered repeatedly by civilian craft on polar routes
- military would not stay in such cold air mass for the duration of many hours unless purposely built
- it could be a case of certification / guaranteed performance / uncharted territory
- higher TAS and Mach do make a difference, for the cold fuel case that's what the QRH drill is

Still sounds a far fetch.

likely made from different thicknesses. Might be an issue of brittleness at the thickness of a Airbus skin.

Although I suspect - as Flight Detent posted above - the reason is that at in the A320F at the limit of -70*C ventilation valves can start to play up. Seen it.

Dave Therhino,

With regards to BA38, I'm not totally convinced of the results of the investigation, extensive as it was. My point being, after having read the final report, with all that was done throughout the investigation I believe one important thing was omitted. That being undercowl "heat soak".

With the aircraft flying for 12 plus hours all engine components are subject to heat soak and with the cowls closed the surrounding undercowl environment will be rather warm and well and truely above room temperature. Therefore the FOHE casing and components should be warmed not only by the hot oil passing through the internals of the FOHE but also from the surrounding heat soak. With the combination of heat soak, hot oil and cold fuel over time, the FOHE should reach a " happy medium" temperature wise and this temp IMHO would be above freezing. This was not done in the investigation process.

From experience from being around an engine when an aircraft has arrived on chocks and the cowls immediately opened, things are well and truely warm to touch and some components too hot to touch. Open to correction, but food for thought.....

Rgds McHale.

My hunch is the other way around, the valves are tuned down to the limit and a little less, once exceed they get confused. A symptom as opposed to reason. BTW, have you not actually also first heard it?

Capt, I was involved in the early portion of the BA38 investigation. Fuel waxing/freezing was one of the very first things we looked at. Short answer is that the fuel never actually got that cold - very cold, yes, but cold enough to cause the observed problems, no. And remember, there was quite a bit of fuel recovered after the accident, so we had solid data on the fuel characteristics.
I was no longer involved by the time they were testing the FOHE icing (I was working 747/767 at the time, brought into the early investigation as a subject matter expert), but I know several people who were and they were quite convinced they'd found the smoking gun. Further, we were able to go back through historical data and find evidence that this wasn't the first time the FOHE had iced up (e.g.otherwise unexplained increases in oil temp during steady state operation) - just the first time it had caused a major problem.

I may well be wrong, I'm working from memory, happy to be corrected etc etc but I thought BA introduced the low temp step down into the flight planning procedure for T7s after the BA38 accident. Whenever it was introduced, and again as I recall it, a descent i.a.w. with the flight plan wasn't mandatory. You descended if the observed fuel temp demanded it, OTOH if the temp didn't demand it you were free to ignore the descent.

The step down built into the fuel plan was to simply make sure you had loaded enough fuel in the tanks to make destination if the step down to a significantly lower level was was needed..

tdracer - thanks for your insight, much appreciated.

tdracer,

Thanks for the feedback, very interesting indeed.

Rgds McHale.

As an aside, I recall flying over Siberia in winter probably about 15 years ago.

We were in a B744 @ FL370 or so. The SAT outside was pretty much standard ISA at about -56degC, the temperature on the ground at Irkutsk (or wherever) was -57deg C!

Yup, Certainly looking at trop level on the Siberian charts some winters days it was clear you'd have to descend a heck of a long way before you saw the OAT increase. I've certainly seen the SAT rise by a degree or two as a result of a step climb when "up north" .