BACK TO BASICS
GET TO KNOW YOUR AIRPLANE
By David Dunteman
Arizona Pilots Association and Recreational Aviation Foundation
Issue 1, January 23, 2016
Can I get Airborne?
“If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.” ― Sun Tzu, The Art of War
How many times have you ventured out to a new location and thought “I know I can land and takeoff there”, and without any further thought, landed?
This may not end poorly when venturing out to long strips at low elevations, but we are setting a dangerous precedent if we don't build healthy habit patterns in our preflight planning. This short newsletter will take a look at the often-ignored POH, FAA requirements for pre-flight planning, effect of weight, runway slope, wind, and elevation on Takeoff distance and present tables for you to use to explore your own aircraft.
What planning am I supposed to do?
We all know we can takeoff from our home field, but what happens when we venture to shorter fields or higher elevations?
91.103 Preflight action.
Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include—
(a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;
(b) For any flight, runway lengths at airports of intended use, and the following takeoff and landing distance information:
(1) For civil aircraft for which an approved Airplane or Rotorcraft Flight Manual containing takeoff and landing distance data is required, the takeoff and landing distance data contained therein; and
(2) For civil aircraft other than those specified in paragraph (b)(1) of this section, other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.
Airport Elevation, Slope, Weight, Wind, Temperature
Rules of Thumb from Imeson’s Mountain Flying Bible
Upslope Takeoff Distance: From 1 degree up to 2 degrees, add 10% per degree to the Density Altitude (DA) takeoff distance.
Downslope Takeoff Distance: Decrease DA takeoff distance by 5 % per degree.
Ground Effect: Fly no higher than ½ the wingspan above
the ground until obtaining Vx.
We would find our planning would have been much more in-depth if we knew this outcome was a possibility! Part of planning
for success involves boring homework.
Homework Project One: Find the POH T/O Distance for your aircraft at your home field elevation at 70 degrees and no wind. Now do the same for Smiley Creek Idaho at 7160 feet elevation and 80 degree’s.
My aircraft is a Cub Crafters Top Cub and here are my values.
1. Home KDVT, 1500’ field elevation and 100 degrees. Weight with half Fuel, 1650 pounds and me. No wind takeoff 425 feet. Same problem with today’s 60-degree temperature yields 320 feet no wind.
2. Smiley Creek yields a 750-foot takeoff roll. If I added full fuel an adult passenger and gear it becomes 1470 feet.
3. There are a couple of points to consider. First, knowledge of the longer takeoff roll should help me to not want to pull the aircraft off the ground in the same amount of time that I normally do at home. The problem is we become creatures of habit and “know” that our aircraft comes off the ground in about a five count at home. We travel to a much higher elevation and now we see the anticipated rush of the ground but if we notice (we usually wont) the airspeed is not where it should be to rotate. If we rotate at this point and try to pull the aircraft off the ground we may make it into ground effect but we will not accelerate very quickly due to the high amount of induced drag with our higher pitch attitude which we pull to in order to try to make the aircraft fly. At best we settle back to the runway and abort. At worst we pull to an altitude higher than our wing span and a wing drops in a stall and we attempt to lift the falling wing with aileron which in turn increases the angle of attack on the already stalled wing which then further increases the drag and decreases the lift on that already down going wing and we are in an unrecoverable incipient spin. This exact accident has happened repeatedly at high-density altitude or high wind locations. We must arm ourselves with the knowledge of truly knowing our aircraft as well as ourselves.
If the above discussion about the incipient spin didn’t make sense to you consider an upset training course.
Determine Density Altitude
For each 10 degrees above or below standard temperature, add or subtract 600 feet to or from the field elevation.
Fixed Pitch Propeller DA Takeoff Distance
To the standard SL takeoff distance, add 12% for each thousand feet of DA up to 8000 feet. Add an additional 20% for each additional 1000 feet DA above 8000 feet.
Constant Speed Propeller DA Takeoff Distance
To the standard SL takeoff distance, add 10% for each thousand feet of DA up to 8000 feet. Add an additional 15% for each additional 1000 feet DA above 8000 feet.
DA Rate of Climb (ROC)—Fixed-Pitch Prop
Reduce SL ROC 7% for each 1000 feet DA up to 8500 feet and 8% for each 1000 feet above 8500.
DA Rate of Climb (ROC)—Variable-Pitch Prop
Reduce SL ROC 6% for each 1000 feet DA up to 8500 feet and 8% for each 1000 feet above 8500.
T/O Distance Varies as the Square of the GW
New weight divided by original weight squared equals takeoff distance factor to be multiplied by the original T/O distance at original weight.
Headwind Reduces T/O Distance
Determine T/O distance for DA. Multiply by 0.90 and subtract the value of the ratio of the headwind divided by the rotation speed.
Tailwind Increases T/O Distance
The Tailwind T/O distance equals 110% of normal takeoff distance plus a percentage of tailwind speed divided by the rotation speed.
Takeoff from Various Surfaces
Increase the Takeoff Distance: Firm Turf—add 7%; Rough, Rocky, or Short Grass—add 10%; Long Grass—add 20-30 %; Soft Field—add 23-75%; Mud or Snow—add 50% or more.