Hellenic aviator

On some of our early model B777s, I notice that I can only insert a max distance of 511 nm on the FIX page.

It was explained this is due to the software...

Having read Bill Bulfer's "Big Boeing FMC User's Guide", it explains that for specific software e.g Product Improvement Package (PIP), Pegasus, 747-400 or 777, it states 'max is 511 nm. (511.9 divided by 2 equals 256. Computer architecture)'.

Having ''Google searched'' I couldn't find any answers.

...I am still left wondering the reason why 511, as opposed to 999, which is what our B777-300ERs allow.


Can anyone give me any further information or answer?

Old Smokey perhaps....?

STBYRUD

It seems to be related to the way the FMC handles numbers internally, 511 equals 1111111 in binary (1+2+4+8+16+32+64+128+256), so 9 "full" bit. Whoever designed it figured that 9 bit would be enough for a distance between waypoints - so the system will not allow anything further apart. If it were to accept it without a change in the underlying code the GC distance would 'roll back', similar to the 2038 problem when the 32bit number counting the seconds since 1970 will overflow on Unix systems. The new 777s probably are only limited by the number of digits (3), so using one more bit (512) will give a theoretical maximum of 1023 which is then artificially limited to 999 for practical purposes. Is that what you were asking for?

Spooky 2

I have never heard of this limitation on any PIP, Pegasus or 777 FMS. As a matter of fact if you put NPOLE on the Fix page it will give you the distance and in most cases where I have flown on the N. Atlantic or No. Pac, it's around 2,000+ miles.

Hellenic aviator

STBYRUD, thanks for your explanation, makes sense to me now...

Spooky 2, it seems to me this is a software issue. I fly often across the Pacific and Polar routes on our B777-300ERs.

Our B777-200s and -300s will only allow a max distance of 511 to be inserted.
Our -300ERs allow up to 999. All our fleet FMCs wont allow NPOLE insertion. The only way around this (for us) is to insert N00E(or W)179 on our FIX page or N000.0E(or W)179.59 on either the FIX or LEGS page.

Spooky 2

Amazing that an operator would be doing polar ops without having the NPOLE in the NDB simply for situational awareness. This is not a big dollar item so you may want to ask your ops folks for this small addition. Wonder what else is missing.

Old Smokey

Quick answer, the FMC programme is written in Octal 3 digit code (Incredible to believe that it's not Hexidecimal).

511 is 8 raised to the power of 3 minus 1, i.e. 8X8X8 = 512 - 1 = 511!

Now, if they'd only used the first three 'digits' of the four available in Hexidecimal, that would allow FFF, of which &H(FFF) = 4096 - 1 = 4095, a much better distances for fixes!

The minus 1, of course, is because all counting begins at zero, not 1.

There's all sorts of number bases available, Octal, Decimal, Hexidecimal etc. In the FMC/LNAV programmes that I write, I use a base of 215, which allows for extremely large numbers to be "crunched" into a few digits of computer code

Regards,

Old Smokey

STBYRUD

Octal eh, why do those guys have to be sooo special?

john_tullamarine

Octal

.. didn't everyone cut their teeth on binary and octal before going onto serious assembler and then higher level language programming ?