I'm studying for my AIRAT (Instructor Written) in Canada using Nizus, but I've always had trouble answering the questions about takeoff distances at 27 ºC when given a chart with 20 ºC and 30 ºC, even since the start of PPL. I want to understand it properly so I can effectively teach my students who have troubles with it.
The question is a two-parter, first calculating a takeoff distance with the following values:
|Airport Temp||30 ºC|
|Airport Elevation||3000' AMSL|
|Wind||10 kt tailwind|
With a pressure altitude of 2089', the nearest values are the 2000' and 3000'. I used the 2000' line of the chart.
At 2000' at 20 ºC the ground roll is 1080' and the total to clear a 50' obstacle is 1895'.
At 30 ºC it's 1155' and 2030'.
The chart says it's configured as follows: 2300 lbs, flaps 10, full power prior to brake release on a paved level dry runway with no wind. The notes state:
Headwind subtract 10% per 9kts
Tailwind add 10% every 2kts up to 10kts
Dry, Grass Runway or Gravel add 15% to ground roll.
At 30 ºC I calculate my takeoff distance being 3305'.
The second part asks by how much the takeoff distance will decrease if the temperature drops to 27 ºC.
I'm doing something wrong here as I calculate it to be 3250', with a distance change of 55', but that isn't an available answer.
How do I get the proper numbers to use at 27 ºC?