View Full Version : 737 FMC Speed Limits
Lord Spandex Masher
18th Jul 2012, 20:31
Bit of a strange one that neither of us could explain.
Having been asked to climb and cruise at .77 by ATC the FO selected .77 on the FMC Limits page (Progress Page 2 IIRC). However, as we climbed higher and the high speed yellow bar reduced to below .77 the aircraft decelerated and the climb speed indicated on the climb page also reduced, we ended up at about .755. However, again, selecting .77 on the climb page was possible and the aircraft then accelerated to .77.
I'm fairly new on type but the FO wasn't and couldn't explain why.
Couldn't find an explanation in the Tech manual, FCOM, or Ops manuals.
Any pointers or explanations about why it would reduce in the first place but then allow you to increase it again? I'm assuming that had we selected the speed on the climb page initially none of that would've happened.
Cannot compute that. Why did it not show 0.82 on Prog 2 anyway (NG)? Changing the limit speed will not on its own make the a/c accelerate. What cruise Mach had you selected? What happened to the yellow bar when you selected 0.77 on the climb page? Why did you not level off earlier to maintain 0.77M? What was the max alt shown v your selected alt?
Lord Spandex Masher
18th Jul 2012, 21:41
Sorry not an NG, 300.
Limits page had min and max speeds, max defaults to .82 and min .4 (don't know if that's a customer option).
Normally using CI 5/6 gives a climb speed of .73 ish. I've used the limits page before and selecting a higher min speed limit does increase climb speed and shows it on the climb page, assuming original climb Mach is less than the min selected.
Cruise Mach was again about .73 due to CI but that was also increased to .77 min on the limits page.
The yellow bar remained at .755 but actual speed increased to .77 when that was selected on the climb page, without changing limits page. At that point we had no speed restrictions but had left it at .77 because we were running late.
Max alt was above selected, 340, but I can't remember exactly how much however, company limit us to having max at least 700 feet above cruise level so it was at least that.
FlightPathOBN
18th Jul 2012, 21:52
What version of software on the FMC?
Lord Spandex Masher
18th Jul 2012, 21:58
10.8
plus some extra words to satisfy the electronic masters.
FlightPathOBN
18th Jul 2012, 22:58
10.8 has quite a few bugs...on a 300, that is, well...10.8 tries to encompass many features that would not be common on a 737-300...
Boeing has issued 10.8A to fix the issues....Boeing was supposed to have 11 out a long time ago, but there were so many bugs, they had to do a 10.8A, 11.0 will be out around Sept
flying apple
19th Jul 2012, 07:05
i'm also new on type but if i would change the speeds i would go for the climb or cruise page
but i noticed if you put it in the climb page you have to put it again in the cruise page
Seems a strange way to crack the nut? As f a says, just increase to .77 on Cruise and descent page. A properly functioning FMC should then warn you about Max Mach and adjust max alt. Are you saying the FMC allowed you to climb at .77 with a 'buffet boundary' of .755? Sounds like an MOR to me.
Denti
19th Jul 2012, 08:16
@FPOBN, i guess it is 10.8A as 10.8 is not an approved FMC version anymore.
Anyway, it is a pretty weird way to change the min speed, it is much easier to just change the speeds on the respective flight phase pages or simply change the cost index which changes all speeds at once. Another way is to use speed intervention if that is fitted of course.
Lord Spandex Masher
19th Jul 2012, 08:33
If it was me I would also just change the climb and cruise pages, we don't have speed intervene, but each to their own. Changing the CI is fine until the wind changes.
But it's not really the method I'm worried about.
I'm more interested why, in one instance, we were limited by the high speed bar and in the other we weren't.
Lord Spandex Masher
19th Jul 2012, 17:28
Nope, nothing wrong with flying faster than the yellow bar, the barber's pole is the limit.
No, 'nothing wrong' as you say, but you are into your 0.3g buffet margin (basically almost any bank will cause buffet) and the FMC should not permit it. Unless I completely misunderstand the FMC logic, it was duff!