**Computing VDP using time and DME**

>>HAT = 350' AGL

Time from FAF to MAP = 3:35

So you take the HAT, chop off the last digit and get 35 seconds. <<

The way you have written this is correct, but doesn't show the principle behind the computation. The rule of thumb you have outlined is used to compute an approximate VDP when none is shown in the IAP. The basis is that, at most final approach speeds, a 3 degree descent angle will result in about 600 fpm descent rate...that is, 10 feet per second. Therefore if you divide the HAA (not HAT, for most NP approaches...) by 10, you will get the number of seconds to subtract from your timing so that you will know an approximate VDP. In the example, if you had not sighted one of the required visual cues for further descent from MDA *or* landing (91.175) by the 3:00 minute point (3:35 - 0:35), you could not reasonably expect to land using a normal rate of descent and normal maneuvers (91.175). This is because, in this example, that 35 seconds at a descent rate of 10 feet per second would be necessary to perform a stabilized, visual final approach from the MDA to the runway.

>>That means that if you don't see the runway environment when you're 35 seconds from the MAP, start climbing because after that you won't be able to make it at a normal rate of descent. Just remember to not actually follow the missed until reaching the MAP.<<

Good points.

>>There's also an easy way to do it for DME....<<

Here is the rule of thumb if you are calculating an approximate VDP and the MAP is based on DME: Each 100 feet of HAT equals 0.3 NM. This is because a descent of 1000 feet for every 3 nautical miles over the ground is approximately equal to a 3 degree descent profile. So, in the example, 350' HAT calls for allowing 0.3 + 0.3 + 0.3 + 0.15 = 1.05 NM. Round it to 1 nm. Here's how you use this 1 NM figure: If the DME to the MAP is based on a navaid that is on the final approach course...that is, if the DME *increases* on final approach, *subtract* the 1 NM from the MAP DME. When you get to this point, you should be able to see one of the required items (91.175); if not, a missed is almost certain. On the other hand, if the DME *decreases* on final approach (that is, the IAP is based on a terminal navaid, *or* a navaid that is on the "far" side of the airport), you would *add* the 1 NM to get the approximate VDP.