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I was playing FSX again lately and was flying the B737-400. So I hope it is simulated correctly and this question isn't dead on arrival :-)

It always bugged me, that the altitude display on the MFD is very ambiguous when you are immediately below a "round" value:

enter image description here

On first glance everyone (ok..me) would read the altitude to be 9900 ft. Only if you know what to look for and spot that the numbers carry on downwards you can deduce that you are actually at 10000. The reason is that the second significant digit does not roll over smoothly but ticks over appruptly when the next round value is surpassed.

Granted, 100 ft may not be a huge difference, but it seems very unneccessary to have it like that. If the first two significant digits behaved more smoothly without this round-down like behavior I would recon that it would be way less ambiguous.

Why is the altitude display designed that way?

Edit: Here is what it looks like in motion:
enter image description here

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I think it is an artifact of the simulator after all; the real display isn't like that.

The way that you describe the behavior of the altimeter tape in the simulator is that

the second significant digit does not roll over smoothly but ticks over appruptly when the next round value is surpassed.

However, in the YouTube video "Boeing 737 cockpit instrument approach landing" (uploaded by aviafilmsproduction on Jan 6, 2015), we can see that the hundreds digit does "roll over smoothly" rather than "ticking over abruptly," so there isn't really any ambiguity.

The video shows an altimeter tape which initially reads 3840 and gradually decreases. The two rightmost digits scroll constantly. After the number reaches 3800, the three rightmost digits all move up together; the "800" part moves out of view and is replaced with "780". So, the altimeter never falsely appears to read 3700 or 3870. Likewise, after the number reaches 3000, all four digits move up at the same time, being replaced with the number 2980.

I'm not sure what the altimeter tape in your screenshot is trying to say; the analog portion (the "ruler" with tick marks) seems to be reading about 9960, while the digital portion (the "odometer") looks like it's reading 9900, and you're saying that it's actually reading 10000. In any case, judging by the video I linked to, in the real Boeing 737, if you're much closer to 10000 feet than to 9980 feet, then the altimeter will appear to read 10000, not 9900.

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  • $\begingroup$ Thanks for the description and video link. I found one climbing as well and there it is the same deal, behaving like you would expect a number "ticker" to work. I'll cross check with the sim later, maybe it was indeed badly designed (or updated in the meantime, because I think its based on ~2005 vintage :-)). $\endgroup$ – Jens Jul 31 at 7:24
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Because thats how old school "steam" altimeters work/look and glass cockpits were designed to work similarly such to reduce unfamiliarity when transitioning. Prior to the screen age a drum altimeter would have read out and increased in much the same way the glass cockpit does now.

enter image description here

(source)

You will also notice the 10's and 1's place's are joined and rounded off by 20's. This has to do with the common arrangement of tick marks on any old altimeter as well as the fact that in a fast climb the one's digit would be unreadable.

On any note, its not ambiguous to a pilot, its actually quite accurate if you know what you are looking at.

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  • $\begingroup$ Too accurate in my opinion. I favor a round dial altimeter specifically because of this. Anything less that spot on gets rounded down, and at first glance you may think you are a thousand feet off. $\endgroup$ – Michael Hall Jul 30 at 22:47
  • $\begingroup$ So with the altimeter pictured in your answer, does the hundreds place “tick over abruptly” when you go from, say, 9,999 feet to 10,000 feet? If so, is there any way for the pilot to distinguish between 9,900 and 9,999 at a glance? If it doesn’t “tick over abruptly” like the glass altimeter in the simulator does, then I don’t think this answers the question. $\endgroup$ – Terran Swett Jul 31 at 1:08
  • $\begingroup$ @TannerSwett here is a video of it in operation, it does not tick over that hard youtube.com/watch?v=kFxfy5wQsq0 $\endgroup$ – Dave Jul 31 at 1:13
  • $\begingroup$ @Dave thanks, yes with this style seen in the video as also mentioned by Tanner Swett it is much better and behaves more like you would with such a number ticker. I'll add a video later when I get home to illustrate how it looks in FSX. Any chance something like this was changed or updated in the real cockpit in the last 15 years? :-) $\endgroup$ – Jens Jul 31 at 7:26
  • $\begingroup$ Please see edit to OP for a video. $\endgroup$ – Jens Jul 31 at 16:42

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