Quick Math in the Air

[flyingtexas]

I fly a 172 and would like to ask if anyone has a quick formula to share on how to determine glide distance and airborne time remaining in an engine out scenario? I roughed one out in my head but wondered if anyone has a good solution that works for them?

 

[flav8r]

9:1 One thousand = one mile
This is a rough estimate for a C172SP and there are many variables the will increase or decrease the distance.

 

[Morne]

AGL / VSI = minutes remaining

 

[upstateny]

Doesn't help for gliding, but another quick math formula.

[Cruising Altitude - Pattern Altitude (in thousands)] X 4 = miles out you should begin descent

(for 500 fpm descent @ 120kts)

 

[AuntPeggy]

9:1 One thousand = one mile
This is a rough estimate for a C172SP and there are many variables the will increase or decrease the distance.

Quickest math and roughly accurate. Wind will make a difference, so keep that in mind. Also, I found that after pitching for best glide, I put my fist on the edge of the glareshield. Anything above my fist is not reachable. Amazing how little distance you will cover.

 

[GeorgeC]

Wouldn't one thousand=one mile be approximately 1000fpm at 65kts? Or is that just padding for the sake of making a conservative estimate?

 

[Steve Foley]

Quickest math and roughly accurate. Wind will make a difference, so keep that in mind. Also, I found that after pitching for best glide, I put my fist on the edge of the glareshield. Anything above my fist is not reachable. Amazing how little distance you will cover.

And once you pick your spot, if it's moving up the glareshield, you better pick another spot.

 

[taters]

Add 2 take away 2 for reciprocal headings

Altitude (3) gives top of decent
Ground speed plus a 0/1000 gives you decent rate required

 

[AuntPeggy]

Wouldn't one thousand=one mile be approximately 1000fpm at 65kts? Or is that just padding for the sake of making a conservative estimate?

The POH for my C-172 indicates a best glide ratio of 8:1. So, 1000 ft of altitude would equal 8000 ft of forward glide. or 1-1/3 miles per 1000 feet of altitude. Yes, we have a fudge factor for mental disbelief of an emergency situation, imperfect physical ability to attain best glide, drag from aircraft imperfections, and maneuvering to the selected field. Wind is extra.

Most importantly, there is no mental math required. 1000 ft = 1 mile. You don't have to think about it. Your mind will be free to fly the plane.

 

[upstateny]

Wouldn't one thousand=one mile be approximately 1000fpm at 65kts? Or is that just padding for the sake of making a conservative estimate?

Since you're coming down twice as fast at about half the speed, one thousand feet = one mile works. My calculation was for a gentle, normal power-on descent.

I use it because too many of my students will cruise along until they're practically over the airport and then go into an ear-popping descent. This makes them plan ahead and also provides a good checkpoint on their route.

 

[flyingtexas]

Gentlemen, this was marvelous discussion thank you. I had calculated in my head 300 fpm approx descent with engine out and 60 mph and since I fly in SE Texas estimating 500 elevation. So I was multiplying 300 x thousands of feet in the air minus 500 and 60 mph over the ground. But if 1,000 = 1 mile at best glide that's easy. I'm going to give it a try!

 

[flyingtexas]

I use it because too many of my students will cruise along until they're practically over the airport and then go into an ear-popping descent. This makes them plan ahead and also provides a good checkpoint on their route.

Yes, I used to do that too and after watching the military channel I realized I was probably a good candidate to fly the Stuka so I started working on my descent procedures....with an engine on of course

 

[flyingtexas]

9:1 One thousand = one mile
This is a rough estimate for a C172SP and there are many variables the will increase or decrease the distance.

David I'm not sure I fully understand the ratio 9:1 and what it's a ratio of. Could you explain it a little more please? Thank you.

 

[midlifeflyer]

David I'm not sure I fully understand the ratio 9:1 and what it's a ratio of. Could you explain it a little more please? Thank you.

for each unit high, you go 9 units lateral. So, if an airplane has a 9:1 glide ratio, it means for every 1000' AGL, your glide distance would be 9000' laterally. Converting the 9000' to NM, that's 1.48 NM.

That's what I do. My personalized checklists has the glide ratio in NM/1000' altitude. No conversion calculations for me. Just simple multiplication.

 

[RotorAndWing]

I fly a 172 and would like to ask if anyone has a quick formula to share on how to determine glide distance and airborne time remaining in an engine out scenario? I roughed one out in my head but wondered if anyone has a good solution that works for them?

When I was teaching in Hughes/Schwiezer 269's we would teach in the event of an engine failure and entering auto-rotation, look at your toes through the bubble, that's where you were heading.

 

[Capt. Geoffrey Thorpe]

Quickest math and roughly accurate. Wind will make a difference, so keep that in mind. Also, I found that after pitching for best glide, I put my fist on the edge of the glareshield. Anything above my fist is not reachable. Amazing how little distance you will cover.

Yes.

Figuring out how many miles you can go still leaves you with the problem of estimating of how many miles away is that inviting looking patch of ground.

 

[dcmoser]

I experimented with glide on my airplane and have a couple wind scenarios charted. I then stuck a small stickum on my side shield and one on my knee board. Yes, wind direction will change but it gives me a starting point.

 

[flyingtexas]

I experimented with glide on my airplane and have a couple wind scenarios charted. I then stuck a small stickum on my side shield and one on my knee board. Yes, wind direction will change but it gives me a starting point.

Would you mind sharing that scenario you put on the stickum?