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Visual Descent Points

Rules of Thumb

Back in the days before VNAV and LPV approaches we were searching for ways to avoid the "dive and drive" technique and came up with visual descent points. Every now and then you will find a reason to compute a VDP.


 

A Visual Descent Point is found by subtracting the touchdown zone from the Minimum Descent Altitude and dividing the result by 300.

Definition

A Visual Descent Point is found by subtracting the touchdown zone from the Minimum Descent Altitude and dividing the result by 300.

Example

images

Figure: Non precision approach, Misawa Air Base, Japan

In my Hawaii Boeing 707 squadron, we often flew into Misawa Air Base, Japan. The "dive and drive" technique was the mandatory procedure back then so we needed to know when to leave the MDA, which was at 660' MSL. Since the runway was at 94' the math came to:

VDP = ( MDA - TDZE 300 ) = ( 660 - 94 300 ) = 1.9  nm from end of runway

Looking at the approach plate it appears the end of the runway is at 0.3 DME, so we would use a VDP of 1.9 - 0.3 = 1.6 DME.

Proof

images

Figure: VDP, from Eddie's notes.

The technique is based on the idea that a 3° glide path comes to about 300 feet per nautical mile, and the math follows easily:

VDP = ( MDA - TDZE 300 )

Let's say, for example, your MDA is at 1300' and the touchdown zone elevation is 100'. The VDP would be found (1300 - 100) / 300 = 4.0 nm from the touchdown zone. If the airport has a VOR located at the approach end of the runway, the touchdown zone is at (750' / 6076') = 0.1 DME on the other side. Your VDP, then, would be at 3.9 DME on the approach side.

Of course there is an error here, because:

Altitude lost in 3° glide path = 6076 sin ( 3 ) = 318 feet

That is only off by 6%, good enough.

More About This:

For more about this rule of thumb, see: Visual Descent Points.

Book Notes

Portions of this page can be found in the book Flight Lessons 2: Advanced Flight, Chapters 11.

Bottom Line

So what about the claims this rule of thumb is based on 60-to-1? My conclusion: It is trigonometry.

Rule of Thumb 60-to-1? Trigonometry π
Visual Descent Point    

60-to-1 — Rule of thumb is based on the mathematical relationship of a 360° circle and/or 6076' to 1 nm.

Trigonometry — Rule of thumb is based on the relationship to a right angle and the derived trigonometric functions.

π — Rule of thumb is based on the relationship of a 360° circle, the number π, and/or 6076' to 1 nm.

Revision: 20151213
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