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# What is the meaning of the A/B etc. in the speed FMC etc. 250B/FL100

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Ok what is the A or B at the end of a speed in the FMC, lets say 250B/FL100 dos this mean Below? and A is Above? any one explane in more detail please ;)

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Ok what is the A or B at the end of a speed in the FMC, lets say 250B/FL100 dos this mean Below? and A is Above? any one explane in more detail please ;)
You are correct. The B means at or below and the A means at or above. IF you see TWO altitudes like this 240/3000 4500, that means that you have to cross over that waypoint between 3000 and 4500.

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Yes I think it is. B is for Below, A is for Above.

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Yes. Exactly what you said. B is below, A is above. So 250B/FL100 is to cross the waypoint at less than 250KIAS at FL100.

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Yes. Exactly what you said. B is below, A is above. So 250B/FL100 is to cross the waypoint at less than 250KIAS at FL100.
It's actually "at or below" and "at or above" - so in your example you could cross right at 250 and it'd still be valid.

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It's actually "at or below" and "at or above" - so in your example you could cross right at 250 and it'd still be valid.
You learn something new every day! Granted, the sim almost never takes it to those extremes. /FL070AFL100B will probably result in crossing in the FL080-085 range...

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Thx gents for reply, but when do you use the extra A/B maybee stupid question but if we go for the 250/FL100 it means that the plane wil hit the marker/point with that exact speed/alt ergo 250/FL100!, why put in the A/B and what wil it do diffrent? Ok the same goes ofcourse for the FL, 250/FL100A etc. ? and how big range do you have from setting A or B at Speed/FL 10-20% range? meaning if 250B/FL100 you can hit the cross ower at between speed 230-250/FL100,if 250A/FL100, you can maybee crossover at 250-270/FL100 ? is this correct?. Same then for FL

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The plane will at least try to hit it at exactly those speeds without a A/B restriction, but you can probably expect +-2 or 3 knots depending on wind, and how you've managed your flight w.r.t. speeds. On a lot of charts, especially for "claustrophobic" TMA's, you see that you have to maintain a certain speed OR BELOW (usually when crossing a specified DME of a point near the airport). For instance, Schiphol TMA has MAX 220 knots within 15DME of the SPL VOR. So when the aircraft crosses 15DME SPL (in the NGX, at least), assuming that point is on your approach procedure, it will have decelerated to 220KIAS or less, if you input 220B. London is also a good example, 250KIAS or less after passing the SLP.

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The plane will at least try to hit it at exactly those speeds without a A/B restriction, but you can probably expect +-2 or 3 knots depending on wind, and how you've managed your flight w.r.t. speeds. On a lot of charts, especially for "claustrophobic" TMA's, you see that you have to maintain a certain speed OR BELOW (usually when crossing a specified DME of a point near the airport). For instance, Schiphol TMA has MAX 220 knots within 15DME of the SPL VOR. So when the aircraft crosses 15DME SPL (in the NGX, at least), assuming that point is on your approach procedure, it will have decelerated to 220KIAS or less, if you input 220B. London is also a good example, 250KIAS or less after passing the SLP.
Yes ofcourse, he he feel stupid, thx Frank for clearing that up :D

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You learn something new every day! Granted, the sim almost never takes it to those extremes. /FL070AFL100B will probably result in crossing in the FL080-085 range...
This all depends on what the other restrictions in the procedure are - a lot of times it does need to hit a block like that at the upper or lower end depending on what precedes it and what comes after it. This is the most complicated part of the FMC's code - figuring out where the VNAV path should actually be when there's a ton of variables involved - the solver algorithms that do this are the single most time consuming thing in the programming of the plane.

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This all depends on what the other restrictions in the procedure are - a lot of times it does need to hit a block like that at the upper or lower end depending on what precedes it and what comes after it. This is the most complicated part of the FMC's code - figuring out where the VNAV path should actually be when there's a ton of variables involved - the solver algorithms that do this are the single most time consuming thing in the programming of the plane.
And i say THANKS AGAIN for make it!!! VNAV is perfect!!! NGX is perfect!!!

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