ILS Glideslope Height
Height an aircraft should be above the runway threshold when established on the ILS glideslope at a given distance.
Formula
Variables
Example
3° slope at 3 nm (18228 ft): h ≈ 18228×tan3° ≈ 955 ft
How it works
An ILS glideslope is a straight-line descent path, so the aircraft’s height above the runway threshold depends on simple right-triangle geometry. The distance from the threshold is the horizontal side, the height is the vertical side, and the glideslope angle tells you how steep the line is, which is why the tangent function is used. This lets you estimate how high the aircraft should be if it is exactly on the published glide path.
When to use it
Use this when checking expected altitude on an ILS approach at a known distance from the runway threshold. It is useful for approach training, cross-checking instrument indications, and quick mental estimates of descent profile.
Worked example, step by step
Given: D = 5 nm and θ = 3° Step 1: Convert distance to feet: 5 × 6076 = 30,380 ft Step 2: Apply the formula: h = D tanθ Step 3: Substitute values: h = 30,380 × tan(3°) Step 4: Calculate tan(3°) ≈ 0.0524 Step 5: Compute height: h ≈ 30,380 × 0.0524 ≈ 1,592 ft Answer: The aircraft should be about 1,592 ft above the runway threshold.
Common mistakes to avoid
- Using nautical miles directly without converting distance to feet when the answer is needed in feet
- Confusing the glideslope angle with a pitch angle or descent rate
- Using sine or cosine instead of tangent for the right-triangle relationship
Did You Know?
The standard ILS glideslope is 3°, which conveniently puts you about 300 ft above the ground for every nautical mile from touchdown.
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