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Descent Rates on Non-Precision Approaches

Source: www.pilotworkshop.com/tips/non_precision_approach.htm, Featuring Bob Nardiello

Bob Nardiello:

"Let's talk about the LDA 2 at Hartford because this particular approach really speaks to the need to know where you are, and slow the aircraft down.

As you'll see, on this approach we've got a number of issues. One is a very short distance between the final approach fix LOMIS and the Missed Approach Point. Actually the Missed Approach Point is about a half mile short of the runway threshold on this approach. The distance is only 5.4 miles.

The intermediate altitude on this approach is 2,200 feet as you approach LOMIS. The straight-in landing minimum is 460 feet. That's a difference of 1,740 feet that you must lose over a distance of 5.4 miles. Looking at the information on the approach plate: if you're flying the approach at a 100-knot ground speed, that's 3 minutes and 14 seconds. That's a descent rate well over 500 feet a minute.

The real problem comes particularly in the summer when the winds are out of the south and you're doing this approach to minimums. It's not unusual to have a 20-knot tail wind on this approach, which means your ground speed if you're flying the airplane at 100 knots would be about 120 knots. That gives you 2 minutes and 42 seconds from the final approach fix to the Missed Approach Point. You'll need a descent rate of over 700 feet per minute average.

Well why is this important? It's important because you need to know what's coming next. When you reach LOMIS you need to be prepared to descend the airplane immediately. Cruising over the top of LOMIS and gradually starting a descent will mean that you either won't reach MDA or you'll have to descend at an excessive rate to reach the MDA. And the reason that's not really desirable is once the descent rates get up around 1,000 feet a minute or so, it becomes more difficult to round out the descent and stop the descent."