Brandon8

I am currently a student pilot and we are now calculating how far on the runway we have to go until we are at rotate. We are flying the Cessna 172N Skyhawk (1978). We also need to calculate how far we have to be able to go until we are able to clear a 50 ft. obstacle.

I am having trouble and don't really understand how to get the answer. Here is a question that I got from class and it would be great if you guys are able to help me out.

Weight: 2300 LBS Elevation: 2000 ft. Temp: +5 Celcius Altimeter: 29.92 Headwind: 18 KTS

Ground Roll = ?
Total To Clear 50 ft. Obstacle = ?

Can anybody help me out?

Thanks a lot!

heckler45

My answers....

2200lbs
Ground roll....648'
50' Obstacle...1194

2400lbs
Ground roll...838'
50' Obstacle...1472'

2300lbs
Ground roll...743'
50' Obstacle...1333

Page 5-14/15 in the 172 Information Manual
You have to interpolate just about everything here.

You are at a pressure altitude of 2000' so that's the easy part. Now you have to use the chart for the two weights 2400lbs and 2200lbs. Now start with the 0C and 10C and add the two on the 2200lbs chart. Then divide the sum. Now you have a headwind of 18kts so you have to decrease your distance by 10% for each 9kts. So the sum you have right now just mulitply by .80 which is a decrease of 20%.

Now do the same thing for for the 2400lbs chart. When you get that figure add the one from 2200lbs and the one from 2400lbs and divide by two. There is your Take off distance. Now just repeat again with the given figures for Clearing 50' Obstacle.

Just remember it's all about interpolation which if you hate math makes flying really unenjoyable. But it gets easier over time. Hope this works out for you.

paca1888

Previous poster said it pretty well. However, if you want a little cheat, calculate density altitude and use that in place of pressure altitude and use the standard temperature column. At least it takes out one interpolation (can be used for any performance charts/graphs). I don't have an N model POH on me but the numbers above sound about right.

EDIT: Actually IIRC the N model POH has examples and specific instructions on how to calculate everything. Cessna actually recommends rounding up to the nearest chart value to calculate performance related items as it adds in a little fudge factor (though this may not be what your ground school wants you to do).

Brandon8

Thanks for the replies.
Heckler, I have a couple of questions for you.

Basically, whatever my elevation is, it will be my pressure altitude? Is that right?

Why would I use the two weights 2400 lbs and 2200 lbs? (I scanned my POH Takeoff Chart for you guys so you can see what I am looking at. My chart doesn't go up to 2400 lbs?)

ImageShack - Hosting :: 48867332gg5.jpg

Also, how do I know which temperature to use on the chart? You used 0C and 10C. That is what my instructor used for the sample question too. Did you use those numbers because the question temperature was 5C?

My instructor had different answers than you; but that's probably because you had different numbers in your POH?

My instructor had:
Ground Roll = 718 ft.
50 ft. Obstacle = 1290 ft.

Thanks,
Brandon.

heckler45

Actually only on a standard day (29.92) would airport elevation be the same as pressure altitude. Just remember that "Pressure Altitude" is "Indicated Altitude" corrected for non-standard pressure.

Quick equation for figuring out pressure altitude...

29.92 - current altimeter = ? x 1000 + airport elevation = pressure altitude.

Example:
29.92(Standard Pressure) - 30.14(Current Altimeter) = -0.22 x 1000 = -220 + 560'(Airport Elevation) = 340' Pressure Altitude.

The 172 "Information Manual" I used was for a 1982 C172P so that may explain why our answers were not the same. The method in which the correct values are obtained are the same though for your ground roll. I used your image you scanned and came up with the same answers your instructor had.

I did use the temperatures for both 0C and 10C in order to obtain the correct value for 5C. But in my first explanation I used 2200lbs and 2400lbs only because in my "IM" there is no chart for 2300lbs. That is where Interpolation came into play.

Just keep with it Brandon, it will get easier. You're on the right track asking questions too. I remember when I first started all of this stuff made aboslutely no sense to me but I just kept doing problems over and over. One day a light bulb clicked on and suddenly everything made sense. Good luck in your endeavours. If you need more help just ask.

Brandon8

Heckler, thanks for explaining everything to me! I'm still in school right now so I can't try the question again but I will when I get home and I will tell you how it goes!

Yeah it's tough right now but I'm only 17 and I am going for my PPL. I started early !

Brandon8

Sorry. I still don't really get it. How did you know that 0C and 10C would give you the correct value at 5C? How do you know not to look at 20, 30, or 40C in the chart?

2300 lbs is in my chart so does that mean I look at just that or do I have to take two weights? Do you only take two weights when the weight stated in the question isn't in your takeoff chart?

Thanks,
Brandon

the King

Simple. Interpolation. It means to use two known values to find an unknown. You'll use a ratio for two known values (distance required for 0C and 10C) and the value you want to know (5C). In this case the solution is right in the middle. You just need to average the values for 10 and 0 and you will find the value that is in the middle, which happens to be 5C.

You are really overthinking the problem. Look for the values from your problem in the table. If you find it, be grateful. If you don't, take 2 values that surround it and interpolate to find the result.

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