﻿ Rules of Thumb # RULES-OF-THUMB

 DENSITY ALTITUDE Density Altitude Computation For each 10º F above (or below) standard temperature at an airport’s elevation, add (or subtract) 600 feet to (from) the field’s elevation. Multiply the field elevation in thousands of feet (e.g. 6,454’ = 6.5, 640’ = 0.64) by 3.5 (lapse rate) and subtract from 59º for standard temperature. TAKEOFF DISTANCE Gross Weight Takeoff Distance The takeoff distance varies with the square of the gross weight. A 10 percent increase in gross weight equals 21 percent increase in takeoff distance. Density Altitude Takeoff Distance Fixed-pitch Propeller: To the standard, sea-level takeoff distance, add 12 percent for each 1,000 feet of density altitude up to 8,000 feet. Add an additional 20 percent for each additional 1,000-feet density altitude above 8,000 feet. At 9,000 feet density altitude (8 * .12 = .96 plus .20 = 1.16). Add 116 percent to sea-level takeoff distance. Density Altitude Takeoff Distance Constant-speed Propeller: To the standard, sea-level takeoff distance, add 10 percent for each 1,000-feet density altitude up to 8,000 feet. Add 15 percent for each additional 1,000-feet density altitude above 8,000 feet. Headwind Takeoff Distance A headwind will reduce the normal takeoff distance for any particular density altitude equal to 90 percent of the takeoff distance minus the value of the ratio of the headwind component divided by the rotation speed. This will be approximately 10 knots = –27%, 15 knots = –39%, 20 knots = –52.5%, 25 knots = –65% Tailwind Takeoff Distance The tailwind takeoff distance is equal to: (a)               110 percent of the computed density altitude takeoff distance, plus (b)               the value of the takeoff distance times the tailwind component divided by the rotation speed. Density altitude takeoff distance * 1.10 = (a) Density altitude takeoff distance * TW/VR = (b) Total takeoff distance = (a) + (b) Runway Surface Takeoff Distance Firm turf – add 7% Rough, rocky, or short grass (up to 4 inches) – add 10% Long grass (4 inches or more) – add 20 to 30% Sand – add 15% Soft sand – add 45% Soft field – add 23 to 75% Mud or snow – add 20 to 50% or more For combinations, add the percentage of each variable. Maximum depth for takeoff in snow or water: ½” slush or water 1” wet snow 4” dry snow Gradient Determine Runway Slope For the runway slope, subtract the low-end elevation from the high-end elevation. Divide the difference by the runway length and multiply by 100. Gradient Takeoff Distance Downslope takeoff distance is reduced (about 5% per degree): 1% downslope – reduce takeoff distance 10% 2% downslope – reduce takeoff distance 11% 4% downslope – reduce takeoff distance 19.5% 6% downslope – reduce takeoff distance 28.5%   Upslope takeoff distance is increased (about 7% per degree): 1% upslope – increase takeoff distance 7.5% 2% upslope – increase takeoff distance 14% 4% upslope – increase takeoff distance 25.5% 6% upslope – increase takeoff distance 39.5% Landing Speed True Airspeed Add 2 percent to the indicated airspeed for each 1,000 feet of density altitude to determine the true airspeed.   Because the air is less dense at altitude, the wings have less lift. Use the same indicated airspeed for landing at airports. The increased true airspeed will automatically compensate. Runway Length Takeoff Distance Use the halfway point of the runway. If 71% of the speed necessary for rotation is achieved, continue, if not, abort. Formula: 10 times the square root of the percentage of liftoff distance required (50% for halfway, square root of 50 = 7.07) is equal to the percentage of liftoff speed that should be attained in that distance (10 times 7.07 = 70.7%). RATE OF CLIMB Density Altitude Rate of Climb Fixed-pitch Propeller: Reduce the sea-level rate of climb 7 percent for each 1,000 feet density altitude up to 8,500 feet and 8 percent for each 1,000 feet above 8,500 feet. Density Altitude Rate of Climb Constant-speed Propeller: Reduce the sea level rate of climb 6 percent for each 1,000-feet density altitude up to 8,500 feet and 8 percent for each 1,000 feet above 8,500 feet. STALL SPEED Stall Speed Increase The stall speed increased as the square root of the wing load factor. 60-degree bank = 2 gs; square root of 2 = 1.41 = 41% increase in stall speed. GROUND EFFECT Ground Effect Fly no higher than one-half wing span above the ground after takeoff until obtaining VX. LANDING SPEED Landing Speed Calculation Add 2 percent to the indicated airspeed for each 1,000 feet of density altitude to determine the true airspeed. Use the same indicated airspeed for landing regardless of altitude. Landing Speed Landing Distance A 10 percent increase in touchdown speed equals a 21 percent increase in landing distance.