# How to estimate the altitude of an aircraft at a given distance from a runway?

What altitude should a heavy class commercial freighter be at on a descent path 17 nautical miles from a runway? (Say SDF airport)

• Which miles are we talking about? Nautical miles (used in aviation)? Statute miles? International miles? – J. Hougaard Jul 2 at 8:09
• International miles. – James Carter Jul 2 at 17:16

Straight in, on glidepath, about 6000' above the runway. Straight-in & fast, below glidepath at 3000 to 4000 feet above the runway.

That's based on about 3 miles per thousand feet is a standard glidepath, and it takes in the ballpark of 5 miles at idle power & level flight to slow from 250 knots to a configuration for starting down the glideslope.

Those are ballpark numbers; a heavy aircraft slows less rapidly than a lightweight one does, headwinds can shorten those distances, while a tailwind at altitude can extend them. And there are techniques to slow a little faster than that, and you can leave the power in & slow down over a (much) longer distance if desired.

But for ballpark values, those are good approximations.

• And crosswinds can impact that somewhat also. – CrossRoads Jul 2 at 4:23
• Nice answer to a somewhat broad question :) – Quentin H Jul 2 at 10:12
• Thanks guys, sorry it was such a broad question. – James Carter Jul 2 at 19:26

The final descent phase is flown at approximately three degrees flight path angle.

$$17 \cdot \tan(3^\circ) \approx 0.89$$

If the aircraft has 17 track miles to go to the runway, and it is on a continuous descent of 3 degrees, it will have an altitude of approximately 0.89 miles above the runway elevation. Assuming nautical miles here, that translates to approximately 5400 feet.

However, it may be that the approach contains horizontal flight segments, in which case the altitude may be lower at 17 track miles to go.

If one is 17 miles from the airport and the aircraft is not flying straight to the runway but has to make one or more turns, then the number of track miles to go will be higher than 17 and consequently the aircraft will be higher.

Instead of estimating, one can use a flight track website such as FlightRadar 24 to get the exact height of the aircraft. Please note that the altitude given on tracking websites is typically derived by a pressure altimeter, uncorrected for the local atmosphere. This pressure altitude is different than the geometric altitude of the aircraft. To acquire the exact (GPS measured) height, a subscription is required.

It all depends... But in many cases, airplanes adhere to specific patterns - and these can differ from airport to airport. While the normal glideslope is around 3° it might vary.

For a start, one might look up the charts of the airport in question.

But at this distance, most planes are not on the glideslope yet - often they capture is from below at somewhat above 10 nm.

But they might also be on a continuous descent - and it is hard to tell by eye at such a distance if the airplane is flying level or descending.

Crosswind and weight do actually have little impact on the glideslope, as in most cases (at least commercial traffic) this is fixed and in most cases, pilots don't deviate from regulations...

Checking the charts for KSDF ILS RW17L (just the one I picked by random) @16.9 NM the altitude would be at or (slightly) above 5000ft MSL when using the default approach.