Mental Math

[Hankpilot2002]

I am gearing up for a Netjets interview and I see that they like to stress some mental math problems; in which I am severely challenged.

I have been computing a VDP by: (MDA - TDZE)/2 such as:

MDA 800 - TDZE 200= 600/3= start your descent 2 miles out for a 300 FPM descent rate.

I suppose this could be expressed as HAT/3=VDP in miles as well.

Is that even a little bit accurate?

 

[Crobar]

td=200'
mda=800'

600' differance, 3 degree glidepath is 300 feet per mile, 2 miles from the threshold is your vdp, either time it or use dme

 

[jet600]

Mental math

As far as VDPs - the easiest way I have found is to take your height above touchdown and divide it by the G/S you want to use...(example...on a non - precision, straight in - lets say HAT= 450' and you want to use a 3 degree G/S. 450 / 300 = 1.5.) Thats 1.5 dme from approach end of Rwy...assuming you have dme ( you may have to add or subtract that number to a current dms already given....clear as mud?) You can also calculate VDPs using time.

 

[gangdev]

From what I understand, VDP is the depicted point on the profile view of an approach plate. The PDP (planned descent point) is more what you are talking about, it can be calculated for time or distance. For distance, use HAT/300. If your HAT is 450, then 450/300 = 1.5(nm). Your PDP is 1.5 nm from the TDZ/runway end. This is based on a 3 degree descent path, which I believe is 318ft/nm, but 300 is close enough for the mental math.

For time, use HAT/10. If HAT is 380, 380/10 = 38, or :38 seconds. If the approach plate has a time (for your category) of 1:45 minutes from FAF to MAP, then just subtract :38 from 1:45 to get :67 seconds from FAF to PDP. This is a very approximate method, and the 10 is based on a certain ground speed. Anyone correct me if I am wrong.

 

[Hankpilot2002]

Crobar thats the easiest way that I have found to do the VDP. Now if I can only figure out descent planning math....VNAV has made me lazy.

 

[Wankel7]

As far as VDPs - the easiest way I have found is to take your height above touchdown and divide it by the G/S you want to use...(example...on a non - precision, straight in - lets say HAT= 450' and you want to use a 3 degree G/S. 450 / 300 = 1.5.) Thats 1.5 dme from approach end of Rwy...assuming you have dme ( you may have to add or subtract that number to a current dms already given....clear as mud?) You can also calculate VDPs using time.

Wait what? Where do you put the G/S in?

 

[JokerFuel]

3 degree descent formula. Take the amount of altitude you have to lose, and multiply that number by three. That sum will give you the distance at which you must begin your descent in DME. Then divide your groundspeed by two and add a zero to it. That will give you your rate of descent. Example: Your aircraft is at FL400. You must cross JFK at 10000 feet, and your groundspeed is 400kts. You have to lose 30000 feet. 30 x 3 = 90. Your descent must begin at ninety miles from the JFK VOR. 400/2 = 200. So you must descend at 2000 fpm

 

[learflyer]

what ever happened to just landing the f*&^ing airplane?

 

[a319stud]

3 degree descent formula. Take the amount of altitude you have to lose, and multiply that number by three. That sum will give you the distance at which you must begin your descent in DME. Then divide your groundspeed by two and add a zero to it. That will give you your rate of descent. Example: Your aircraft is at FL400. You must cross JFK at 10000 feet, and your groundspeed is 400kts. You have to lose 30000 feet. 30 x 3 = 90. Your descent must begin at ninety miles from the JFK VOR. 400/2 = 200. So you must descend at 2000 fpm

does that really work? been doin this stuff for quite awhile and thats the easiest way i have heard. anybody else heard of this?

 

[JokerFuel]

It absolutely works, I learned it on the Brasillia many years back. Next time you have an assigned descent just use this formula to back up the Auto/VNAV and you will see.

 

[CaravanMan]

Thanks for this thread, guys. Yeah, gee-whiz VNAV stuff has dumbed me up a little bit too! These are handy tricks, so keep 'em coming.

 

[B +]

2nd Segment

Another area that NJA usually asks questions about is second segment climb. The trick to quickly calculating the required climb angle is to take the rate of climb required at 100 kts and divide by 100. This will give you the angle you need. With this angle you can consult your AFM charts to determine if your aircraft can meet this climb angle.

For example if the table on the departure chart show that at 100 kts your rate of climb needs to be 650fpm then you know the required climb angle is 6.5 degrees.

Good luck on the interview.

 

[outerfix]

3 degree descent rate

The wag about descent rate and when to start the descent is half true. The descent rate is accurate for a three degree descent but you need to start the descent earlier. If you needed to lose 30,000 feet you would need to start the descent at 100 miles out.(300 fpnm x 100mi = 30,000)

 

[BengalsFan]

Another area that NJA usually asks questions about is second segment climb. The trick to quickly calculating the required climb angle is to take the rate of climb required at 100 kts and divide by 100. This will give you the angle you need. With this angle you can consult your AFM charts to determine if your aircraft can meet this climb angle.

For example if the table on the departure chart show that at 100 kts your rate of climb needs to be 650fpm then you know the required climb angle is 6.5 degrees.

Good luck on the interview.

So what do you do if you have never thought about this stuff before because your VVI is pegged out and you don't look at trouble T's?

 

[imacdog]

Because your VSI usually isn't pegged after an engine failure.

 

[BengalsFan]

But I only have 1 engine, so it doesn't really matter

 

[B +]

But I only have 1 engine, so it doesn't really matter

You are right with only one engine you do not need to worry about your climb rate after an engine failure. You will have things to worry about but climb rate will not be one of them.

This thread is on the subject of mental math help for the NJA interview. Anyone interviewing at NJA should understand 2nd segment climbs and if you can quickly calculate the angle using this simple rule you will impress the interviewer. I was ask this on my interview.

As a side note I predict the Bungals will give the Dolphins a late Christmas present on Dec 30. Miami will get their one and only win of the season thanks to Cincinnati.

 

[gunfyter]

what ever happened to just landing the f*&^ing airplane?

its nice to know that after you pass the VDP on a non-precision approach without seeing the runway ... you can count on executing the missed approach once you reach the MAP. No need to be surprised since you made your decision already when you passed the VDP.

 

[Wankel7]

Any good mental math for the ...

Your 20 miles out from a crossing restriction and have to lose 10000 in that distance?

that includes GS?

 

[gunfyter]

GS/60 X Altitude/Dist = Rate of descent

You are flying a jet so you are close to:

360 6 miles per minute
420 7 mles per minute
480 8 miles per minute
540 9 miles per minute
600 10 miles per minute

take the altitude divide by the distance and multiply by the miles per minute.

Citation X 9 miles per minute X 10000ft/ 20 miles

9 X 500 = 4500 feet per minute

CJ 6 miles per minute X 10,000 ft / 20 miles

6 X 500 = 3000 feet per minute...

to descend 10000 feet in 20 miles.