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5 Answers

Takeoff Performance Chart Calculations

Asked by: 6819 views
Aerodynamics, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

I need the help from someone involved in light aircraft instructing. In small aircraft like Cessna 172s or Piper 160s, the pilots' handbooks include, of course, takeoff performance charts. These charts indicate takeoff distances based upon gross weights, OATs and pressure altitudes. The charts are simple enough to interpolate from; however, I have doubts concerning their thoroughness. Can anyone tell me if the derived takeoff distances are based on the "implied" density altitudes based on the various temperatures given, or are the derived takeoff distances based solely on pressure altitudes? It does make sense that the distances would imply density altitude performance given that the charts include various temperatures. Nonetheless, the distances themselves seem pretty optimistic. Should a pilot formulate his takeoff distance from the chart and then refigure for density altitude, or should he expect that density altitude is already accounted for? In the larger aircraft that I instruct in we must figure both numbers and base our balanced field lengths and climb performances only upon density altitude. To those of you actively flying 172s and Piper Cherokees, do these numbers in the takeoff performance charts seem plausible? Are they fairly accurate?

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5 Answers

  1. Sam Dawson on Mar 15, 2013

    These are essentially DA charts- you enter at a PA, then adjust for temperature.
    Having said that, I do adjust the numbers I get from these charts. I have an article on my website that explains my technique:

    Basically I remind pilots that these numbers are derived in a brand new airplane with no dings, cracked fairings and probably no bugs on it; with a brand new engine and propeller; flown by a test pilot. So the numbers in the real world, in a real airplane, with a real pilot who may not be at the top of his/her game may not be quite the same. So I recommend adding a safety margin- normally about 1.5 x the book numbers in ideal conditions.

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  2. Dave Sandidge on Mar 16, 2013

    Sam, I have entered your website into my favorites list. I believe you and I must think very much alike when it comes to new students and basic aircraft performance training. When I think about all the planning that goes into one of my company’s flights compared to the lack of planning that occurs everyday in the GA world I quickly understand why there are so many accidents in GA flying. And I believe the key to reduced accident rates is better training in two distinct areas: Aeronautical Decision Making and Aircraft Performance. On the subject of density altitude let me ask you this question – just to get your opinion. Suppose you figure your takeoff on a particular day from an airport at an elevation of 4,200′ to have a density altitude of 7,500 feet. So you start down the runway in your Cessna 172 with a density altitude of 7,500 feet. Would you agree that the airplane will perform at that particular time and place like it would at an airport at an elevation of 7,500 feet with standard conditions – that is to say, the altimeter at 29.92 and an OAT of about 0 degrees C?

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  3. Sam Dawson on Mar 16, 2013

    Yes. Although I have students figure this so they have an idea of how the airplane will perform it is not really necessary with most performance charts as they are essentially DA charts. I have seen a few in older airplanes that actually used DA.

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  4. Mark Kolber on Mar 18, 2013

    Following up on Sam’s post and excellent article, one of the things I put in the information pages of my personalized checklists is a table of takeoff performance for various density altitudes in no wind conditions. Especially when the POH charts are line graphs, it makes for a quick and easy reference.

    I limit the data to takeoff and not landing performance for two reasons: Nothing I fly has a longer landing than takeoff distance. I don’t plan to land at an airport I can’t take off from.

    If the numbers are at all close, it’s time to hit the real charts.

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  5. Dave Sandidge on Mar 21, 2013

    Mark, that’s a good idea. Getting the students in the habit of checking the takeoff distance required is not only one item demanded by the FAA for every departure, it’s also very smart. My heart sinks every time I read in a NTSB report of a family, or group of friends, or just innocent, unaware passengers being wiped out when they crash on takeoff because of a too high density altitude/overload situation. There’s no reason for it; the numbers are there in plain site. It all funnels back to basic aeronautical decision making.

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