> >
>

# Need some help regarding Cessna 172 performance

Private Flying LAA/BMAA/BGA/BPA The sheer pleasure of flight.

# Need some help regarding Cessna 172 performance

24th Mar 2015, 05:37

Join Date: Sep 2014
Location: Earth
Posts: 27
Need some help regarding Cessna 172 performance

Basically I'm trying to figure out the ground roll feet and total feet to clear 50 ft obstacle when pressure altitude is at 1000 and temperature 15 degrees with the use of this chart. Can someone give me an answer with a thorough explanation as to how they came up with the answer thank you in advance

http://imgur.com/qeoRUgS
24th Mar 2015, 06:17

Join Date: Jan 2008
Posts: 602
Your result will be "close enough" if you average the values for each at 10 and 20C. So, ground roll 1010 and 1090; 1010 + 1090 = 2100 / 2 = 1050 feet ground roll. I'll let you do the climb to 50 feet distance calculation using the same method.
24th Mar 2015, 07:19

Join Date: Sep 2014
Location: Earth
Posts: 27
Yes I understand that method but it's not accurate I do recall my instructor showing me another method but reading his hand writing is a bit difficult maybe I'll ask him tomorrow thanks for your input though.
24th Mar 2015, 08:16

Join Date: Jan 2008
Posts: 602
Yes, there probably is a very precise method for interpolation. If the exercise is to apply the method correctly, and the performance values are just the examples, then yes, you'd best perfect the method.

If. on the other hand, the exercise is to determine the required operational distances for the 172, the level of precision could get to be excessive. All kinds of factors will create operational variability of several percent, at least, so having a distance to 50 feet, with a precision of 10 feet is not worth anything other than the exercise of making the calculation correctly.

We do have to learn to use the performance information, but it's more important to spent the time learning how to optimize the aircraft performance by flying it well.

I was asked to take a 182 amphibian, at 3350 pounds GW into a 300 meter long runway. I paced off the runway first, and assessed the winds and approaches, in a preplanning, no rush environment. I reviewed the performance charts, and factored in the variables (wet grass, downhill slope). I had the required distances available. I flew in and out with extra care and attention, but no difficulty. For the landing, the charts said that I had had 40 feet more space available than the specified ground roll distance. For takeoff, a little more. In both cases, I easily used less than the specified distances... (though for stopping, I had special helpful system )

24th Mar 2015, 09:17

Join Date: Dec 2014
Location: Aberdeen
Posts: 43
I am a student pilot but we did this at a ground school class the other week. If you look in the POH would will find worked examples of how to work this out. We were using C172 and C152 examples but I suspect the 182 POH has something similar in it.
24th Mar 2015, 10:11

Join Date: Nov 2010
Location: Tamworth, UK / Nairobi, Kenya
Posts: 613
"it's not accurate"

You're kidding, right?
How "accurate" do you need?

And how "accurate" are you willing to risk your life on?

I would never interpolate, I'd always use the higher value. Then add 10% for whatever, then add 10% for the condition of the runway. (I've never seen a runway that was in "perfect" condition)

I have yet to run off the end of a runway, but I have seen people run off the end of the runway, then tell everyone that the "chart said we'd make it".
24th Mar 2015, 10:31

Join Date: Jan 2008
Posts: 602
Some of the relevant wording from the design requirement for the aircraft (any bold is mine):

(f) Unless otherwise prescribed, in determining the takeoff and landing distances, changes in the airplane's configuration, speed, and power must be made in accordance with procedures established by the applicant for operation in service. These procedures must be able to be executed consistently by pilots of average skill in atmospheric conditions reasonably expected to be encountered in service. (g) The following, as applicable, must be determined on a smooth, dry, hard-surfaced runway--

(1) Takeoff distance of Sec. 23.53(b);
Takeoff performance.

(a) For normal, utility, and acrobatic category airplanes, the takeoff distance must be determined in accordance with paragraph (b) of this section, using speeds determined in accordance with Sec. 23.51(a) and (b). (b) For normal, utility, and acrobatic category airplanes, the distance required to takeoff and climb to a height of 50 feet above the takeoff surface must be determined for each weight, altitude, and temperature within the operational limits established for takeoff with--
(b) For normal, utility, and acrobatic category airplanes, the speed at 50 feet above the takeoff surface level must not be less than:
From this the determination of takeoff distance can be understood. My experience has been that by applying yourself, both to properly factoring in the conditions, and using optimum skill, you can achieve at least the POH results.

But...

If you're planning up to the edge of the performance numbers, where safety could become a factor if you miss it, be very careful! Always have an out - an undershoot or over run area, an alternate departure path, time to wait for better winds, cooler temps, or, two trips, with the load shared for lighter departure weights.
24th Mar 2015, 11:48

Join Date: Nov 2010
Location: Tamworth, UK / Nairobi, Kenya
Posts: 613
So your recommendation to a (clearly) brand new pilot, is to expect to meet the POH numbers?
24th Mar 2015, 12:14

Join Date: Jan 2008
Posts: 602
So your recommendation to a (clearly) brand new pilot, is to expect to meet the POH numbers?
Once competent to fly, and flying in accordance with the POH, yes, expect to meet those numbers. But..... with the warning against flying in an environment in which failing to achieve that performance would increase risk.

The POH does not come with two sets of performance values, one for new pilots, and one for skygods, The information presented is factored for "average skill". New pilots are encouraged to use those values. But, like anything, when you're new at it, give yourself some extra room....

If you apply maximum power very early in the takeoff, and achieve it, in a properly configured aircraft, you would probably have to apply effort to not achieve the height at 50 feet value as presented in the charts. There's a bit of skill in it, but it's mostly just physics....

As an aside, but on the theme, the 172 does come with two sets of numbers for spins. The more restrictive "Utility" C of G limits for intentional spinning, are for average pilot skill. At the "normal" C of G limits a safe spin recovery is still possible, but it will probably require skills beyond that of a new pilot.
24th Mar 2015, 13:40

Join Date: Jun 2002
Location: CYYC (Calgary)
Posts: 5,256
The more restrictive "Utility" C of G limits for intentional spinning, are for average pilot skill. At the "normal" C of G limits a safe spin recovery is still possible, but it will probably require skills beyond that of a new pilot.
ST,

Could you expand on that? It's a long time since I read a 172 POH, so I had a quick look at one for a 1978 172N. I saw a fairly standard spin-recovery technique for spins in Utility category but nothing else.
24th Mar 2015, 14:02

Join Date: Jan 2008
Posts: 602
Spencer, please forgive me, I'm going to drift your thread a bit, but we'll still discuss 172's

The 172 is approved to be spun in the utility category, which category has a more restrictive C of G range than normal category. That demonstration of spin recovery (up to six turns) was made with "average pilot skill". In essence, you're not allowed to have people in the back seat for intentional spins, which seems totally reasonable to me. For this configuration, there is a POH spin procedure. There is also the expectation that you could execute a less than perfect recovery, and there is enough "room" that you're not going to exceed a limitation. (The average pilot skill thing).

The 172 (like all "normal category" certified single engine aircraft (+- the Cirrus?)) has also demonstrated compliance with the requirement that it be recoverable from a one turn spin in not more than one more additional turn - at the full C of G range. Indeed, it probably is the spin recovery characteristics which define the aft C of G limit. However, that requirement does not require just average pilot skill - it can be demonstrated with some practiced effort. And, the recovery could take the plane alarmingly close to a limitation or two, as long as they are not exceeded.

I can say from first hand experience, that larger Cessnas can approach Vne, and at more than 2 G recovering from the dive resulting from a one turn spin. Not territory to be fooling around in without a G meter!

So, in that way, the 172 has two different limitations, based upon how it is to be flown.

In deference to Spencer's original question, (and respect of thread drift) was you question answered to your satisfaction?
24th Mar 2015, 17:04

Join Date: Jan 2004
Posts: 3,581
Originally Posted by Step Turn
Once competent to fly, and flying in accordance with the POH, yes, expect to meet those numbers. But..... with the warning against flying in an environment in which failing to achieve that performance would increase risk.

The POH does not come with two sets of performance values, one for new pilots, and one for skygods, The information presented is factored for "average skill". New pilots are encouraged to use those values. But, like anything, when you're new at it, give yourself some extra room....

If you apply maximum power very early in the takeoff, and achieve it, in a properly configured aircraft, you would probably have to apply effort to not achieve the height at 50 feet value as presented in the charts. There's a bit of skill in it, but it's mostly just physics....

.
I agree with Step but would add one small caution. The POH numbers were achieved with a new properly rigged aircraft. After 20,000 hrs of TLC from students and a "that still looks OK, so we will leave it for now" maintenance regime the aircraft may not be as sprightly as it once was.....

However the point is still valid. In practice I think the missing point is just that "practice" . The main runway at my home drome is 7000 feet long. I see lots of pilots who are happy to land at some random point somewhere along the way. Takeoff's are similarly treated with the same level of casualness.

To meet the book runway length required numbers you have to fly the book speeds. If your approach is 10 kts too fast and you are 50 feet too high crossing the fence the airplane is not going to stop in the book distance.

You don't want to be "practicing" your landing on a minimum length field, you had better show up already "practiced". For landing the secret to making the book numbers is an accurate touch down point at the correct speed. The good news is you can practice that on every landing.

For the actual calculations; well to pass the exam you have to do all the extrapolating. In the real world, as was recommended, just go to the next higher set of numbers and add a fudge factor. 10 % was recommended, although I tell new pilots to start with an extra 25 % until they get a bit of experience.
24th Mar 2015, 18:25

Join Date: Jun 2002
Location: CYYC (Calgary)
Posts: 5,256
ST,

I would like to pursue the topic of C172 spins further, so to avoid further hijacking of Spencer's thread, I've started a new thread:

Spinning a 172

Last edited by India Four Two; 24th Mar 2015 at 19:09.
24th Mar 2015, 20:42

Join Date: Dec 2001
Age: 74
Posts: 31
Take-off performance

As a Pilot Examiner I expect a candidate on a flight test to accurately calculate the distance, interpolating for pressure altitude and temperature as necessary. Once they have calculated the distance my first question is 'will you conduct a take off under these conditions.' The answer I expect is NFW. I expect them to elaborate by stating that they would add additional distance for a number of factors.
This way I am getting them to provide an accurate calculation AND apply principles of Pilot Decision Making.
24th Mar 2015, 21:01

Join Date: Jan 2008
Posts: 602
The answer I expect is NFW. I expect them to elaborate by stating that they would add additional distance for a number of factors.
This way I am getting them to provide an accurate calculation AND apply principles of Pilot Decision Making.
'Best answer I've heard in a long time!
24th Mar 2015, 22:07

Join Date: Nov 2000
Location: Cambridge, England, EU
Posts: 3,437
You don't want to be "practicing" your landing on a minimum length field, you had better show up already "practiced". For landing the secret to making the book numbers is an accurate touch down point at the correct speed. The good news is you can practice that on every landing.
Well, I'm not that good, I'm afraid.

For example I can do crosswinds (to some limit), and I can do short runways (to some limit), but I'm really not keen on trying my maximum tolerable crosswind and my minimum tolerable runway length at the same time - I'll go somewhere else that day. If I'm concentrating on a crosswind at the limit of my ability I'm not really very interested in having to worry about runway length at the same time.

I do fly for fun, after all.
24th Mar 2015, 22:50

Join Date: Jan 2008
Posts: 602
For landing the secret to making the book numbers is an accurate touch down point at the correct speed. The good news is you can practice that on every landing.
I think the suggestion is that you can practice every landing, you don't have to succeed in the short distance, or the spot landing, just work toward it....

The sky, and the interface with the earth, don't know if you're flying for fun, or the most serious pursuit. They demand the same attention in any case!

The only landings during which I'm not practicing some skill, are those occasional ones when I have to pee, and just want to be down and clear with the least commitment....
24th Mar 2015, 23:01

Join Date: Jan 2004
Posts: 3,581
Originally Posted by Step Turn
I think the suggestion is that you can practice every landing, you don't have to succeed in the short distance, or the spot landing, just work toward it....
Exactly. You don't have to be even trying for a short landing, just pick a spot on the runway where you want to touch down. If it works great, If it doesn't ask yourself why. Was the approach too high, too low, too fast, too slow, and then try to work on the factors that caused you to touchdown early or float past your intended touchdown point. The goal is accurate flying just becomes a habit
25th Mar 2015, 09:25

Join Date: Jan 2008
Posts: 602
I bought my C 150 with the STOL kit installed already - the first one in Canada ('cause truthfully, who would spend the money for the kit on a 150 anyway?). I delighted in challenging myself to precise, short landings. The flying club where I'd learned in 150s a decade earlier had, at the time, two intersecting runways, such that from the start of the pavement on one to the centerline of the intersection one was a touch over 200 feet (the first runway has since been lengthened).

I used to entertain myself, on a day with a bit of wind, touching down right on the edge of the pavement (or maybe a foot or two of gravel) and clearing on the intersecting runway without actually sliding tires. It wasn't long before the chief flying instructor took me aside and didn't ask, but told me to stop doing that. I was setting a poor example, as students would watch me, and try the same things in the brand new 152's, which were just not capable.

So, I learned the various patches and marks on the runway, and did the same thing, just much further up the runway, to the next intersection.

These days, I've mostly grown out of that kind of behaviour, but I'll stop short of calling it immature. The skills I have developed, have been helpful from time to time.... Suffice it to say, the landing distances I could achieve were much shorter than the POH tables!
25th Mar 2015, 10:15

Join Date: Apr 2012
Location: Perth, WA
Posts: 327
ST, I did a similar thing, quite innocently. I learned to fly on a C150 in the bush and, by the time I got my PPL, the idea of safely assessing the minimum landing speed was well drummed in. When I moved to the big smoke, and flew the first few times from the city GA airport with my first aircraft - a C150F - I was annoyed to sometimes see all the aircraft blocking the runway 'exit'. No harm..just taxyed to the next one. It was a week or so before a friendlier-than-average controller suggested that, unless I wanted a clearance to the helipad, I might want to give it a bit more distance.

I'm more sophisticated, but probably lazier, these days

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off