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.

I first learned about this rule of thumb at the Air Force Instrument Instructor's Course (AFIIC) where we were told it is derived from the 60-to-1 concept. But is that true? Well let's find out. But first, we'll define the rule, provide an example of its application, and then we'll provide a mathematical proof of the rule.

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

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

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:

$\mathrm{VDP}=\left(\frac{\mathrm{MDA}-\mathrm{TDZE}}{300}\right)=\left(\frac{660-94}{300}\right)=1.9\mathrm{nm\; from\; end\; of\; runway}$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.

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:

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:

$\mathrm{Altitude\; lost\; in\; 3\xb0\; glide\; path}=6076sin\left(3\right)=318\mathrm{feet}$That is only off by 6%, good enough.

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.

Not every instrument approach is designed with stability in mind, in fact, some cannot be flown safely as depicted. See Approach Impossible for more about this. The first clue that an instrument approach may not have your health and safety in mind is if the math from the Final Approach Fix to the runway is greater than 318 ft/nm — the classic 3° glide path — it might work out, but it might not. Here is an example.

Another problem for computing a good time to descend — a VDP — is for a circling approach. It can be done, but it isn't clear cut.

Dear Eddie,

The Van Nuys RWY 16R ILS is out of service for a while and we are now being issued the VOR-B when the weather is good, the VOR-A when it isn't. ATC will not approve a Circle Maneuver East of the airport because of KBUR ILS Rwy 8 Arrival flow / KBUR Rwy 15 Departure flow (Aircraft make climbing right turn towards KVNY). The airport is extremely difficult to find and especially so at night as you are not lined up with a runway and you are also not looking directly at the airport. I am flying a Challenger 605, which is Category D for Circling.

So for the VOR-B, the VDP is 3.2 NM and my target VS is 1,226 ft / min or call it 1,300 ft / min. Thus, if I do not see the runway before ZEXUG I am going to have to go missed and ask for the VOR-A or ILS RWY 8 KBUR VOR-B.

I am concerned about pilots wanting to make it work from the VOR-B and being even more encouraged to do so with the circling limitations. This combined with a shortened runway makes me feel like too fast, too steep, diving approach, with long landing on a now shortened runway will result in an over run accident soon.

Thanks.

Signed, R. Fader

Fort Lee, New Jersey

Dear Mister Fader,

You are right to be concerned, this looks like a bad situation at night or when the visibility goes down. First of all, as depicted, the approach cannot be flown on centerline with any hopes of being stable. When faced with such an approach, I like to draw out the "planned" descent rates. The person who drew the approach planned it just to get it done within the letter of any law that can be found in TERPS. Here is an example where "just because it is legal doesn't make it safe."

The only thing on this drawing you cannot get from the approach plate is the distance from the VOR to the touchdown zone (TDZ). I plotted that using GoogleEarth and got 0.6 nm.

As you can see from my drawing, You are much higher than a stable approach. This is what I call a "Flounder Approach." That comes from the movie "Animal House" where Bluto tells a pledge named Flounder, "Face it Flounder, you screwed up. You trusted us!"

To illustrate the math, I'll work out the first part of Option 1 for you right here. The altitude to lose is 3,580 - 2580 = 920 feet. The distance to lose that is 2.5 nm. So the required descent rate is (920 feet) / (2.5 nm) = 368 ft/nm. From your letter I gather you were doing the approach at 130 KTAS, so you will need a vertical velocity of (130 nm/hr) (368 ft/nm) (1 hr / 60 min) = 797 ft/min. Here is how it looks:

- Option 1 (As published) PURSY to ZEXUG — 368 ft/nm (797 ft/min),

then ZEXUG to TDZ — 574 ft/nm (1,244 ft/min) - Option 2 Spot runway at PURSY, intercept 3° Glide Path at ZEXUG — 684 ft/nm 1,482 ft/nm)

then 3° glide path — 318 ft/nm (689 ft/min) - Option 3 Spot runway at PURSY, fly to TDZ — 482 ft/nm (1,044 ft/nm)

That descent rate just 3 miles from touchdown is unsafe!

Shoving the nose over for nearly 1,500 fpm at 5 nm might not be too bad, but it isn't good.)

In my opinion, if you haven't spotted the runway by PURSY, you should go around. If you do spot the runway at PURSY or sooner, you should maneuver east to pick up the ground track of the ILS and pick up a 3° glide path as soon as possible. You could even pick up a 3.5° glide path (as published for the ILS), the math is the same except that instead of descending at 318 ft/nm, you will be descending at 371 ft/nm.

You mentioned the VOR-A as the low ceiling option. It could be that if you cannot spot the runway from that direction it is a nonstarter. But if you can, it may actually offer a better chance at a stable approach. The key point to remember is that you want to roll out on final 1.57 nm from the touchdown point to provide a stable approach at 500 feet AGL. Doing the math, you need to start down about a quarter of the way through the turn. I don't expect anyone to go through all this math, but you just need to envision that your target on rollout is 500' AGL.

So if this was a day approach underneath a distinct ceiling with good visibility, the VOR-A could be a good option. But notice that the approach's viability minimum of 3 statute miles is only 2.6 nautical miles and I compute that you need to begin your turn at 2.75 nautical miles. I would be very tempted to bypass all this and head for Burbank.

Signed, Eddie

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

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