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How would you know what altitude to be at to capture the glideslope on an autopilot RNAV or ILS approach? (I am mainly focused on MSFS airliners, but this could apply to other planes or in real life)

When you are too high, the autopilot does not descend for the final. It just flies straight over the runway and disables approach mode. Sometimes, you set the autopilot's altitude to 3,000 feet, and it works. Sometimes, not. This can be annoying, because you then think your autopilot has a problem. Well, how do you fix this?

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  • $\begingroup$ Yes. That's what I meant. When you are too high, it will still fly the approach, as if it was going to descend, but you have to be below the altitude constraint for the waypoint to actually capture the GS. So basically it just flies above the runway and does not descent at all. $\endgroup$
    – DaCuteRaccoon
    Commented Sep 17, 2023 at 17:47
  • $\begingroup$ Is this a home flight simulator question? In real instrument flights, the approach chart has the crossing altitude for each waypoint and the connecting segment will never have a slope greater than the final glideslope so the only physical option is to intercept the final slope "from below" $\endgroup$
    – Max Power
    Commented Sep 17, 2023 at 21:18

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The best way to find the platform altitude, is to look at the respective approach chart. For example, if you want to fly the ILS approach into runway 09L at London Heathrow, you can go to the UK AIP, navigate to "Part 3 AD 2 Aerodromes", select London Heathrow (EGLL), go to section AD 2.24 and open the chart for INSTRUMENT APPROACH CHART ILS/DME I-AA RWY 09L - ICAO:

EGLL ILS 09L Approach Chart

I marked the platform altitude in red: 2500 ft AMSL. So you should be at 2500 ft when passing DME 7.5 on the localizer to intercept the glideslope.

Note that ATC might clear you to descend to a lower altitude. For example, you might get vectored onto the localizer closer to the runway and ATC could therefore clear you to e.g. 2000 ft instead. In absence of such an instruction, you should maintain 2500 ft until intercepting the glideslope.

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  • $\begingroup$ Yeah but on simulators the charts don't work (I have tried using one and it didn't capture the GS) and ATC sucks. The best way for SIMs is the other solution, but yours is for sure the better one for IRL. $\endgroup$
    – DaCuteRaccoon
    Commented Sep 17, 2023 at 17:55
  • $\begingroup$ @DaCuteRaccoon Yeah, sim ATC is useless, I agree, unless you try VATSIM, IVAO, etc. But flying the approach according to the chart should work exactly like in real life, as long as your sim is realistic enough... $\endgroup$
    – Bianfable
    Commented Sep 17, 2023 at 18:33
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    $\begingroup$ @DaCuteRaccoon So go purchase a set of instrument procedure charts. In the USA a paper book for a large region of the USA is $8 at any small-airport pilot shop or they can be ordered online. Or you can get them free on the FAA website (and a "chart users guide") skyvector.com is a graphical way to select airports and then get a list of proceedures. Low level IFR en-route charts are helpful for getting to an initial approach fix. In other countries or if you just prefer the format, you may need or want Jeppesen charts. There are tablet apps paid or free that can be used to manage all this. $\endgroup$
    – Max Power
    Commented Sep 17, 2023 at 21:31
  • $\begingroup$ Oh and charts update every month(for legal use when flying) but for simulator practice, about 95% of the data doesn't change in a year and far fewer changes in a month. The bigger problem will be the simulator being based on old data and approaches that have been removed from the current publications. $\endgroup$
    – Max Power
    Commented Sep 17, 2023 at 21:36
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When you learned how to do an ILS approach, they probably said to fly lower than the glideslope. Usually, this is somewhere around 2,000 feet. But it can vary. A solution could be setting the autopilot altitude to the lowest setting, but, well, you're just not supposed to fly for 10NM at 100 ft agl.

The most common solution is to view an aeronautical chart of the area. You can see the altitude, there, but it's hard and I don't recommend this. I don't even know how to do it, as I have never had a problem that needed that (I use the sim, but if you fly IRL, please look at the charts).

The solution for SIM users, is a button. On some planes, there is a button labeled "CSTR." That shows constraints. Little purple numbers will appear on your map if you have configured your approach. On every waypoint, there will be one. If it has a line under it, e.g. "

    2000
" that means you have to be 2000 feet OR ABOVE. No lower than 2000 feet. If it has none, it means you have to be below that. Usually it's the waypoint right before the runway. If it says "1700", then after you pass the one right before that and are headed for the "1700" one, then set your target altitude to 1700, and THEN turn on approach mode.

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  • $\begingroup$ Hint: CSTR is Airbus specific; on a Boeing, press the DATA button to see the constraints. You can also view them on the (M)CDU flight plan page (shown on the right next to each waypoint). $\endgroup$
    – Bianfable
    Commented Sep 17, 2023 at 12:55
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    $\begingroup$ @Bianfable I'm not familiar with a "DATA" button on the 737 MCDU. Is that what some Boeings use in place of the "LEGS" page / button? Or something else specific to other Boeing jets? $\endgroup$
    – Ralph J ♦
    Commented Sep 17, 2023 at 17:49
  • $\begingroup$ I don't think "DATA" is on the 747 MCDU. I am pretty sure, well I can send a picture to where it is on my Airbus. $\endgroup$
    – DaCuteRaccoon
    Commented Sep 17, 2023 at 17:51
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    $\begingroup$ @RalphJ I meant the DATA button on the EFIS controller, which is AFAIK similar to the Airbus CSTR button to show times and constraints on the ND next to each waypoint. $\endgroup$
    – Bianfable
    Commented Sep 17, 2023 at 18:31
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    $\begingroup$ @Bianfable I'm following you now... yes that button on the EFIS control does exactly that. I don't use it much because, to my eye, it clutters up the display (and I typically go to the LEGS page for altitude constraints), but that also shows them, on the map. Thanks for the reminder! $\endgroup$
    – Ralph J ♦
    Commented Sep 17, 2023 at 20:15

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