Non precision approach question...?

Question

In non precision approaches there is always a decent angle/glidepath and the TCH which is the height of the aircraft on the runway threshold if maintaining the glidepath but i just wanted to know what is the formula for calculating the rate of decent to maintain the glidepath on IAP's.

EX: http://155.178.201.160/d-tpp/1403/05889RF.PDF
The RNAV(GPS)-F at KASE.

What is the formula to keeping a 6.49 Glidepath?


Im not a pilot but im hoping to be one day and im just educating myself on my own at the moment.

Answer 1

(1) The TCH (threshold crossing height) is only applicable to PRECISION approaches. It's not the height of the aircraft ON the threshold, it is the height of the aircraft ABOVE the threshold.



(2) While there are published gradients on non-precision approaches, because there are often step down fixes leading up to the final Approach Fix (FAF) and a constant descent angle often is only applicable from the FAF inbound, the simplest way to calculate the required minimum descent rate is to divide the distance from the FAF to the MAP by the estimated ground speed (approach speed compensated for wind). You can also make the same calculation for any of the intermediate stepdown fixes.



(3) You can learn all this and more by downloading a FREE PDF copy of the "Instrument Flying Handbook" from the FAA website.

Answer 2

As the others have stated, a non-precision approach will have step-down altitudes; but are often included on an ILS, LPV or other precision approach plate to conserve paper and for the sake of pilots so they don't have to lug around an even heavier load of charts. I didn't look at the link, but you will often see the angle of glide slope / gradient and it is there for the precision approach. Based on your ground speed during the approach you use the angle to figure what FPM indicated on the VSI will be needed to remain constant on the glide slope. A simpler way is to refer to the chart printed on the back section of all FAA and DoD approach plates. Not sure if Jeppesen still does or not.

Answer 3

Rate of Descent = Glide angle/60 x Ground Speed/60 x 6080

Answer 4

There is no predefined "glidepath" on a non precision approach - you decend to the MDA as rapidly as practical, and start looking for the airport. If you see the airport you land visually - but you do not go below the MDA unless you see the airport and can complete the approach visually.

If you don't see the airport before reaching the missed approach point, you have to execute a missed approach.



Edit: I just checked your example - the approach into Aspen is not typical. They mention the TCH of the VGSI (a visual glide slope indicator), which is not offically part of the instrument procedure, because this approach requires a rather step descent after the FAF.

After the final approach fix you have 5 nm to descend 2000 ft to reach the MDA. If you had a 120 kt ground speed thats only 2.5 minutes, so the vertical speed would have to be at least 800 fpm. Once at the MDA you would need to reduce that to a normal approach,and would have the option of using the VGSI as guidance.



Because of the terrain and altitude, this approach might be challanging to light plane pilots not familar with mountain flying - many Cessna drivers never climb to 13,700ft, let alone start an instrument approach at that altitude.

Answer 5

There isn't one. Look at the minimums, this is a visual approach -- something to get you through the clouds to a roughly 2500' ceiling.