Take off roll data

Murdoughnut

Well sized member
Generally speaking, how accurate is POH takeoff ground roll data? Here's my situation - taking four people (myself included) on a short trip on Sunday in my 172P. We'll be taking off at max gross and landing at a field with a short strip (2,300ft). My takeoff roll calculations (with expected density altitude) come to about 1,500 feet according to the book (excluding wind).

The only other times I've taken off at max gross were during periods of cooler weather - this will be at around 87 degrees. Part of me says just to trust the numbers, but for some reason I still have some uncertainty. How much should I trust the book? A/C is a 1982 model - 160hp BTW.

Thanks in advance
 

Rocketman99

Frozen Guppy Manipulator
How good are your short field procedures? I would assume the POH numbers were run on a plane in brand-new condition with everything working in tip-top shape... If you were within like 2 or 300 feet I'd definitely be seriously concerned. At least you have a little more than 1.5 times the estimated length you need. Just make sure you run your numbers right. If you do have wind, bonus!
 

B767Driver

New Member
Generally speaking, how accurate is POH takeoff ground roll data? Here's my situation - taking four people (myself included) on a short trip on Sunday in my 172P. We'll be taking off at max gross and landing at a field with a short strip (2,300ft). My takeoff roll calculations (with expected density altitude) come to about 1,500 feet according to the book (excluding wind).

The only other times I've taken off at max gross were during periods of cooler weather - this will be at around 87 degrees. Part of me says just to trust the numbers, but for some reason I still have some uncertainty. How much should I trust the book? A/C is a 1982 model - 160hp BTW.

Thanks in advance
Have you flown the airplane heavily loaded before? Not only the weight, but also the CG could alter the handling characteristics from a lightly loaded airplane.

The first time I took four passengers in a Warrior, after mostly training and solo flights as a student, it really surprised me how different the airplane performed and handled with the aft CG.
 

Murdoughnut

Well sized member
How good are your short field procedures? I would assume the POH numbers were run on a plane in brand-new condition with everything working in tip-top shape... If you were within like 2 or 300 feet I'd definitely be seriously concerned. At least you have a little more than 1.5 times the estimated length you need. Just make sure you run your numbers right. If you do have wind, bonus!
Yeah, I usually use 1.5x the calculated distance as my control buffer - this comes in at right about that (based on estimated conditions). I'm pretty confident with short field - and fortunately the runway has nothing but water on both ends - otherwise I wouldn't even be considering it.
 

Murdoughnut

Well sized member
Have you flown the airplane heavily loaded before? Not only the weight, but also the CG could alter the handling characteristics from a lightly loaded airplane.

The first time I took four passengers in a Warrior, after mostly training and solo flights as a student, it really surprised me how different the airplane performed and handled with the aft CG.
Not this aircraft - but other 172s. I took off from a 2400 ft. strip with four people once before, with plenty of room to spare, but the temperature was 10-degrees cooler. CG shouldn't shift too far from the middle.
 

unclenobby

Well-Known Member
Obviously you need to consider all the variables and not just the numbers in the book. It's good you have a 1.5X fudge factor.
Aircraft condition, pilot ability, wind direction and strength, density altitude (hot and humid in Florida not good for perf!), rwy condition and also are there any obstacles to clear (just noticed your response on the water at both ends - does this apply to both airports you will visit?)

I personally wouldn't fly at 172/warrior etc with 4 on board (most I'll go is 3). Since you have a 1.5X fudge factor on rwy lenghts it makes sense to have a fudge factor for aircraft weight as well (don't go over 80% of useful load for example). I also wouln't fly with 4 souls on board at MGW out of a 2400 ft strip even if it was within the numbers, unless i was getting paid to fly, had paying customers on board, and I was very proficient. This is just my opinion,
Let us know what you decide to do and how it works out. (this is a good lesson in ADM!)
 

Baronman

Well-Known Member
Sounds like you're on the right track to coming to your own conclusion. The wind will help you (potentially), but just make darn sure you're not taking off with a tailwind.

Also consider standing water (if there is any). I can't remember if the Cessna POH has a factor to add for water but even if it doesn't, you should make a mental note of it.

Finally my suggestion (and maybe you already thought of it) would be to take less fuel. I know many people are accustomed to topping off the tanks, but if there was a way to measure less you could reduce your takeoff weight by a bit. That was pretty easy in the Piper aircraft because they have a little tab inside the tank letting you fill to a lower - predetermined amount.

Also are you using the actual weights of the occupants (or a good guesstimate?)

Enjoy, sounds like a fun trip
 

Murdoughnut

Well sized member
Sounds like you're on the right track to coming to your own conclusion. The wind will help you (potentially), but just make darn sure you're not taking off with a tailwind.

Also consider standing water (if there is any). I can't remember if the Cessna POH has a factor to add for water but even if it doesn't, you should make a mental note of it.

Finally my suggestion (and maybe you already thought of it) would be to take less fuel. I know many people are accustomed to topping off the tanks, but if there was a way to measure less you could reduce your takeoff weight by a bit. That was pretty easy in the Piper aircraft because they have a little tab inside the tank letting you fill to a lower - predetermined amount.

Also are you using the actual weights of the occupants (or a good guesstimate?)

Enjoy, sounds like a fun trip
Yeah, I'm going to make a game time decision based on the conditions present. Density altitude, if like today, will be around 2,000 feet. Winds will likely be light and variable. I'm actually only taking off with 30gallons for the 2 hour trip - that will put me right at max gross, and drop my weight by around 50 lbs by the time I get there. The first leg is leaving St. Petersburg - have plenty of runway and clearance there. We're going to Cedar Key, though. Although the strip is only 2300', there are no obstructions on either end. I do have precise weights for everyone on board.

I've seen short field t/o procedures recommend either 0 or 10 on the flaps - I would think 10 would be the best option to get the plane in the air quick, but wanted to make sure. Our checklist says 0, but others Iv'e seen say 10.
 

tgrayson

New Member
I've seen short field t/o procedures recommend either 0 or 10 on the flaps - I would think 10 would be the best option to get the plane in the air quick, but wanted to make sure. Our checklist says 0, but others Iv'e seen say 10.
The 160 version should say 10. Regardless, the only reason why an aircraft might recommend 0 is due to the climb gradient after takeoff, since flaps will reduce it. However, this is not a factor in your case.

Flaps always will reduce the ground run in a takeoff if you leave the ground at a lower airspeed. If you rotate at normal speeds and leave the ground at normal speeds, flaps won't help you a bit.

When taking off on a short runway, it's a good idea to have a point in mind where you'll abort if you haven't achieved flying speed.
 

Murdoughnut

Well sized member
The 160 version should say 10. Regardless, the only reason why an aircraft might recommend 0 is due to the climb gradient after takeoff, since flaps will reduce it. However, this is not a factor in your case.

Flaps always will reduce the ground run in a takeoff if you leave the ground at a lower airspeed. If you rotate at normal speeds and leave the ground at normal speeds, flaps won't help you a bit.

When taking off on a short runway, it's a good idea to have a point in mind where you'll abort if you haven't achieved flying speed.
Thanks - I appreciate the info! What I think I'll do is look at how far my roll out is leaving St. Pete (instrument markings will give it away). That should give me a good idea of what to expect. 10 on the flaps for sure, though!
 
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