ISA Deviation page in 737 FMC - Quick question

[Teroc]

Anyone know why on the fmc in the 737 ng only lets you go to minus 16 in the ISA deviation page ?

[Old Smokey]

I'm not B737 rated any more, but here are a few GUESSES which might inspire further research -

Guess #1 : What is the lowest temperature for the environmental envelope of the aircraft? It would surprise me if colder than ISA-16 was not acceptable at lower levels, but most environmental envelopes "go vertical" at higher (e.g. cruise) levels, where a lower limit of ISA-16 sounds feasible.

Guess #2 : It could be a Hexidecimal / Octal problem in the FMC programme, and 16 is the magic number (although F=15, we are talking negative numbers). Hexidecimal / Octal limitations do show up annoyingly in other areas of FMC performance, e.g. for the (Boeing) FMC that I use, the greatest arc that may be drawn is 511 nm radius, check that with Octal and the &H function of your calculator!

Just a couple of guesses to inspire research, standing by for responses from REAL B737 people!:ok:

Regards,

Old Smokey

[Capt Chambo]

First off I don't know the answer, but I had a little play with one of ours today, a -900 set up to FAA performance FWIW.

Actual ISA dev. was -2C, and it would accept up to -20 C. This didn't change the optimum altitude, but changed the max alt by 300 ft.

OS is probably correct as he usually is, but I would hazard a guess that it's a certification issue to ensure that you can't "fool" it into trying to climb to a too high an altitude by falsifying the temp. I would also hazard a guess that a CAA/JAA/EASA performance certified aeroplane, would be more conservative than one certified to FAA standards.

Interested to hear from others.

[Teroc]

Thanks Guys...very informative. Knew it'd be complicated !!

[FCS Explorer]

just in case and maybe:
concerning the DESCEND forecasts page
the ISA-dev is NOT for the landing field wx, but for the entire descent.....
....and entering anything here will change pretty much nothing

concerning the PERF INIT page:
entered the TOC-temp sometimes when it was new. as a result i got crappy max crz calculations, where, during climb the max crz FL would increase by 100ft for every 1.000ft we climbed.
so: it's dashes and not boxes -> just leave'm emtpy

[CaptainSandL]

I tried it today on a planned 6000ft ferry flight and the lowest temp it would take was ISA-67C which equated to OAT-64C.

http://www.b737.org.uk/images/fmc_perf_init.gif

If you look at the environmental envelope in the AFM it shows the minimum OAT in flight at 6000ft is about -65C so I suppose that the FMC has a lookup table for the graph.

http://www.b737.org.uk/images/environmentalenvelope.gif

S&L

[Old Smokey]

Thanks for the Environmental Envelope CaptainSandL, if you were at high altitude (where I implied in Guess#1 in Post #2 that the envelope "goes vertical" at higher levels), therein lies your answer. Above 36089 feet, the lower Temperature limit for the Environmental Envelope is .......... ISA-16°C:ok:

What was the Level at which you encountered the ISA-16 "cutoff" Teroc ?

Regards,

Old Smokey

[Teroc]

Good question Smokey and I dont know the answer...yet.
I'll try it 2moro from the cruise and see what happens...