HIGH DENSITY TAKE-OFFS ....
HIGH-DENSITY TAKE-OFFS ....
Acknowledgements: Thomas P. Turner (Mastery Flight Training Inc.)
(Ed. Note: Thomas publishes a couple of his readers’ submissions regarding a topic covered in a previous bulletin, the substance of which is appended below).
Reader and high-elevation pilot Stanley Stewart writes:
“There is a reason why many Flight Schools require a high-density altitude checkout prior to a pilot flying to a high airport. High-density altitude take-off training can be performed at lower elevations with 50% to 60% power take-offs, but the proper leaning techniques for a high-density altitude take-off must be also taught.
A high-density altitude take-off is very different from, and nothing like, a sea level take-off in several very important ways.
· The engine must be leaned to best power prior to take-off .... lean for maximum RPM with carburetted engines with fixed pitch propellers; fuel injected engines should be leaned to sea level take-off EGT. With two qualified pilots, the pilot not flying could lean the engine during the take-off roll.
· The airplane must not be rotated beyond a shallow climb attitude and be allowed to lift off on its own, when ready to fly in a shallow climb attitude, at a higher indicated airspeed than a sea level take-off, to avoid a high drag producing high angle of attack that could prevent the airplane from being able to climb out of ground effect.
After lift-off the airplane must be flown in level flight (not climbed at all) in or just above ground effect until the maximum rate of climb speed is attained, then the maximum rate of climb speed must be strictly maintained.
Do not expect to see, and do not pitch up to, a climb attitude as steep as at sea level, and do not expect a rate of climb as high as the POH states you should get at that density altitude. The POH figures were developed with professional test pilots flying new, clean airplanes.
Never perform a high-density altitude take-off towards any obstacles or rising terrain.
Avoid intersection take-offs. Full-length take-offs provide more margin for wind shear, etc.
Don’t be tempted to expect POH take-off distances. Add a minimum of 10% because that data, as I wrote above, was developed with professional pilots flying new, clean airplanes.
.... the above procedures must be adhered to with any normally aspirated piston engine airplane. Not writing here anything you do not already know!”
Reader Andy Smith contributes a personal experience and his subsequent analysis of it:
“I wanted to share a story and provide the opportunity for our community to discuss non-standardised take-off performance ....
I was in a Cessna 172N at Sedona, Arizona getting ready to depart. Weight and balance were well within envelope; airport elevation is 4831 feet, and the AWOS reported 29°C; winds less than 5 knots. From the Pilot’s Information Manual, I used 5000 feet and 30°C to determine a 1435-feet ground roll and 2595 feet over the 50-foot obstacle height, with a runway length of 5132 feet.
Most departing traffic had used Runway 21 so far that day because Runway 3 is upslope (1.8%), but the Airport Chart Supplement notes this runway should be used in calm wind. There was traffic on left downwind for Runway 3, another aircraft calling 2 miles east to cross and join left downwind, and a third aircraft 8 miles out and inbound.
(Debrief Part 1: Would you wait until traffic showed a break to take off on the down-sloping Runway 21 as most other aircraft did that morning, or take off on Runway 3 as directed by the Airport Chart Supplement and interface with local traffic?)
I elected to take off using Runway 3 from intersect A8 per the published procedure. I set 40 KIAS no later than taxiway A5 as my abort point, leaned for best power on the runup per normal high altitude take-off procedures, and waited for a break in traffic .... My friend with a PPL about one week old was flying and I was monitoring instruments, calling for flight following, and reading checklists. While we did break ground at 55 KIAS, we struggled to accelerate to a normal VY or climb rate while in ground effect. I was worried we miscalculated weight and balance, and would be unable to maintain flight after the end of Runway 3. I immediately made the decision that if we made it to the end of the runway but failed to break ground effect I would turn right into an area of descending terrain, but in the event that was unnecessary.
Personal analysis:
1. At a high-density altitude airport, we should have used full runway length, even if it meant waiting longer for traffic.
2. The Pilot Flying seemed to pull the airplane into the air rather than setting the appropriate take-off attitude and letting the aircraft fly off. I believe this degraded acceleration and climb rate performance immediately after rotation.
3. While we accounted for density altitude and checked performance, it turned out to be unconservative. The only thing I did not include in my performance analysis is the upsloping runway as there was no POH guidance. This certainly played a part in the visual illusion that we weren't climbing as quickly as expected (despite climbing, the ground was chasing us a little too).
4. After checking nearly every information resource they all agree (as do I) that up-sloping runway on take-off & down-sloping on landing will extend distances. There is plenty of regurgitation of the physiology and visual effects of sloping runways but not about performance calculations. And the POH gave no guidance either.
I finally found one source that had the following rules of thumb:
· An upslope increases take-off distance by approximately 7% per degree.
· A downslope reduces take-off distance by approximately 5% per degree.
· A downslope increases landing distance by approximately 10% per degree.
Debrief Part 2: Do you calculate take-off and landing performance on every flight, even if it's a familiar airport? When you are faced with something like my situation - no direct guidance in the POH, no previous training, and no known FAA or industry reference material, how do you make the safe/unsafe determination?”
FLY SAFE!