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Thread: How to avoid accidents taking off to clear an obstacle and how to measure the height

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    PRA Secretary JOHN ROUNTREE 41449's Avatar
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    Lightbulb How to avoid accidents taking off to clear an obstacle and how to measure the height

    There was another accident while trying to clear an obstacle!

    Here is the preparation I have used to pre-flight taking off from non-airport to assure I can clear an obstacle at the end of the road or farmers field.

    1. Look at you Altimeter for the field's altitude. Hopefully you have recently set it with flight following. If you have a GPS it will provide it too you.
    2. Take note of the surface of your runway. Short grass, tall grass, dirt, soft sand, etc.
    3. Pace off the distance from the start of the runway to the obstacle.
    4. Measure the height of the obstacle. It easy to do. You only need a pencil or stick.
    5. Simply hold the stick up with your arm held stretched straight out parallel with the ground and sight the top of the stick or pencil so it is at the top of the obstacle.
    6. Now move you fingers to the base of the obstacle so the length of the obstacle is measured by where you are holding it at the base of the obstacle and the top of the pencil at the top of the obstacle.
    7. Next you simply turn the pencil from the vertical to the horizontal sighting where your holding it is still at the same base of the obstacle where you measured the height.
    8. Now simply note where the top of the stick is now pointing you on the ground. Make sure your arm is still stretched out the same as when you measured the height. If you have another person around have them stand where it points to the left or right of the base of the obstacle, depending which way you turned the stick to the horizontal.
    9. Then just pace off the distance from the base to where the person is standing or the rock/bush etc that you noted where the top of the stick pointed on the ground.
    10. That is the height of the obstacle. As you are just creating a triangle and measuring the base.
    11. Look up the distance in your POH (Pilot Operating Handbook) and see if you have enough distance to take-off and clear the obstacle taking into account the wind, field surface, and density altitude.
    12. I like to do a false takeoff just to lift off and note where that is on side of the field to pace that distance off later and abort the takeoff this allows you to make sure the surface, density altitude etc matches the POH's numbers. If not I need to adjust the distance required.


    Note: Also know that this pilot probably could have cleared this obstacle but did not have enough training!!!

    In the video they are taking off down a slight slope and probably have some tailwind component also. The decision to give the slope precedence over the wind is correct. However, they are probably near (or over?) the MTOW (maximum takeoff weight) as well as taking off on a rough, uneven grass surface. There are obstructions at the far end which can cause unpredictable turbulence just when you need it least.

    In this case, I would not have held the stick all the way back while trying to accelerate on the ground. This would have been a good case for keeping the stick near or at the forward stop until reaching about 40-50 km/h. DON'T ATTEMPT THIS IF YOU HAVEN'T DONE THIS BEFORE AND KNOW WHAT YOU'RE DOING, THOUGH. If done improperly, this is a perfect setup for blade flapping.

    And once in the air, keep your attitude as shallow as possible using slight turns to steer clear of potential obstacles until you have the required speed (Vx) to climb.
    Last edited by JOHN ROUNTREE 41449; 05-12-2016 at 07:17 PM.
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    PRA Secretary JOHN ROUNTREE 41449's Avatar
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    Interpolating temperature and calculating density altitude.

    Many pilot have learned that when you do not know the actual pressure of the atmosphere from a control tower or flight following you may set your altimeter to 29.92 the standard pressure to get a guess of the farmers field or roads altitude you are taking off from.

    While pressure altitude provides you a correction for pressure and you can calculate the altitude it does not correct for the temperature above or below the standard temperature of 59 degree Fahrenheit = 15 degree Celsius. So that alone is no help in knowing your density altitude it only tells you the altitude of the road or farmers field!

    Calculations of pressure is done by calculating the difference between the current altimeter setting and the standard setting of 29.92 and the difference is then converted, based on the given that 1" Hg = 1000 feet.

    We can do a rough interpolation for temperature but not density altitude as pilots know that for every 1000 feet the temperature lapse rate is 1.98 Celsius we round up to 2 Celsius degrees = 3.56 Fahrenheit per 1000 feet higher of lower and there is a formula will give you a better than interpolation guess.

    I'm not going to type the actual formula to calculate density altitude as even I do not memorize it as you can easily spin the wheel of your EB6 (flight computer) and it has both Celsius and Fahrenheit scales you just need to know the altitude and temperature.

    If I have a GPS I just use its altitude and I know the weather report before I fly and spin the wheel is the easiest way to discover your density altitude. If really high elevations I may adjust the temperature lapse rate by 2 Celsius degrees = 3.56 Fahrenheit per 1000 feet but that has never happen in real life.
    Last edited by JOHN ROUNTREE 41449; 05-12-2016 at 07:07 PM.
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