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Effect of air temperature on take off distance

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  • Effect of air temperature on take off distance

    Newish pilot here, flying the ubiquitous and in my eyes lovely C42’s. J

    I’m interested to know, is there a general rule of thumb or graphs available for the effect of temperature on take-off distance?

    The C42 doesn’t have any performance charts that I can find and although it’s possible to work out things like pressure and density altitude, it would be nice to have something approximate for those hot days when you’ve got a large passenger, a fullish tank and a shortish runway.

    Does - increase the distances by 25% on a hot day (35C) sound about right? That would be 125m instead of 100m take-off distance for the C42 100 hp version.

    All advice welcome and thanks..

  • #2
    Hi Richard, welcome to microlighting. The simple answer to your basic question is 'yes'. The higher the temperature the higher the true airspeed you need to achive to get lift-off, the longer the take-off roll, and the lower your climb rate. You also need to take into account the effects of wind variations and the fact you won't be flying as accurately as the test pilot who worked out the figures in the POH, quite possibly using short field technique in ideal conditions. You should add an extra safety factor... 30% is commonly quoted.

    I looked up the C42 POH on the internet and found this:

    Yours should be similar. Note the distance is given to achieve 15m height - this is normal, as is the statement that it is at maximum take-off weight of 450kg. They quote over 200m for the take-off. If you consider your take-off profile (ground roll, lift-off, accelerate, climb) you might be off the ground after your quoted 100m distance, but you need another 100m to get into a safe climb.

    I see the POH also refers to the CAA Safety Sense leaflet on Aircraft Performance. This has a simple reference chart at the end - I recommend it.

    Keep safe


    PS. my rule of thumb for "for those hot days when you’ve got a large passenger, a fullish tank and a shortish runway" is to stay on the ground till it's cooled down in the evening and maybe drain some fuel from the tank. The paperwork involved after you've been rescued from a tree is horrendous. As a 'newish pilot' you might find this safety sense leaflet interesting reading:
    The pilot formerly posting as MadamBreakneck
    R examiner and TST pilot.


    • #3
      Thanks Joan, the reference chart at the end of the Safety Sense leaflet was just what I was thinking of. That will be kept somewhere handy.

      Being 'newish' with about 70 hours of flying I'm very much aware of trying to be a better pilot. The accident reports for the C42 make interesting reading. They all appear to be, and undoubtedly were, entirely avoidable situations but then the pilots involved would have thought that as well.

      PS - I hope you are not speaking from experience when it comes to the paperwork having been rescued from a tree.


      • #4
        The higher the temperature the higher the true airspeed you need to achive to get lift-off, the longer the take-off roll, and the lower your climb rate.
        This statement is correct but I think it is worth emphasising that we don't actually need to start calculating TAS figures that will be required to get us into the sky. It is absolutely true that our TAS will be higher but our take-off IAS (I'm not getting into the semantics of EAS/CAS etc) will be just the same as it is on a 'standard' day. To make sure there is no confusion; you still take-off at the same speed on your airspeed indicator that you normally would, it is just (a)your TAS will be higher if it's toasty outside and (b)it will take a lot longer going down the strip to achieve that same IAS.

        I'm not trying to split hairs but just wanted to make sure there is no misunderstanding about a requirement to calculate TAS to get airbourne.


        • #5
          And of course, inter alia, the engine will develop a bit less power because of the less dense hotter ambient air.
          Other "gotchas" include :Runway slope however slight, grass condition and possible loss/blanking of head wind approaching any line of trees or big hedge.


          • #6
            Thanks for the replies. I agree the difference between IAS and TAS is maybe not so interesting, the important parameters being the increased take off roll and distance to 50’. The summary table puts the different factors of weight, temperature and altitude, etc. nicely into perspective.

            I feel the C42 with 100 hp is a relatively powerful machine, certainly when I see some old GA planes struggling to get off ‘our’ 600 m grass strip and the ultralights fairly rocketing up into the air. What you notice pretty quickly with such a lightweight machine is the difference between solo and two-up performance and I think flying with a ‘large’ passenger is something to be treated with a deal of respect.

            You just can’t get around the physics, which I guess makes flying fun.


            • #7
              My view would be that if I need to start thinking about the difference a hot day makes, the strip is already too short for comfort.
              At any point in the takeoff, I'd like to have either enough runway left to land again, or enough altitude to clear any obstacles and land in an adjacent field.
              Pete T.

              "A closed mouth gathers no feet".


              • #8
                IIndeed, no need to calculate TAS and the rest and sorry if my comment read that way (thank you Paul for picking that one up). What you will notice is that not only does it take more runway to reach the speed to rotate (so the treees at the end will look bigger by then) but it will also take longer (in time) to get to that speed. Ditto the acceleration in ground effect before entering the climb. It can seem to take for ages. You also might want to consider your engine failure after take-off (EFATO) options too - hotter and heavier mean you'll be lower as you go over the hedge, trees or whatever.

                I approve of, and use, the gliding practice of including 'eventualities' as the last check before opening the throttle for take-off: effect of wind on take-off, abort point, engine failure options (ground roll, shortly after lift-off, initial climb out, climb to cruise), and so forth; even at a familiar field and certainly at an unfamiliar one (or if heavier and/or hotter than usual).

                Personally, I've found one of the biggest challenges mentally is sticking to the planned abort point if it's taking "a little bit" longer than planned to reach lift off speed. So far I've not had to be rescued from trees, though one passenger still has memories of a steep avoiding turn at tree top height after a late go-around. That was exciting!

                'll resist geting pedantic about 'ultralight' v. 'microlight'

                The pilot formerly posting as MadamBreakneck
                R examiner and TST pilot.


                • #9
                  Originally posted by

                  I'll resist geting pedantic about 'ultralight' v. 'microlight' [SIZE=8px
                  Sorry. I fly in Germany where microlights don't exist and the the place is full of ultralights. My bad..