FLYING Magazine
Has density altitude affected your flight plans recently? Do you plan for it when you create those summertime flight plans?
I ask because as I write this column Seattle is under a heat advisory. Temperatures are expected to reach into the 90s—for us that’s hot. We wear our Birkenstocks without socks. Lattes are served over ice. Even the salmon in the streams are sweating.
For pilots, the heat rising means there will be an increase in density altitude, that theoretical altitude that your aircraft performs at when flying in high temperatures, humidity, or at an airport that is at high elevation. Density altitude reduces aircraft performance—sometimes dramatically.
To understand density altitude, remember this—the higher the field elevation, the less air pressure; the warmer the air is, the less dense it is. If there is high humidity, there will be more water molecules between the air molecules, and you can expect less performance from the aircraft. Remember the saying “high (field elevation), hot (temperature) and humid,” and note it only takes one of these to create a density altitude situation.
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The theoretical methods for determining density altitude are taught in ground school. This week pilots in the Pacific Northwest will have a chance to experience the real deal. They will learn how to lean the mixture on their airplanes for best performance under the watchful eye of their instructors. Apps will be used to crunch performance numbers. POHs will be referred to and mechanical E6-Bs deployed to make sure the learners have multiple ways to determine those critical performance values so they do not fall victim to “it-should-be-OK” syndrome.
The FAA offers a great deal of information on the effects of density altitude. Check out FAA-P-8740-2, AFS–8 (2008) HQ-08561 and the Pilot’s Handbook of Aeronautical Knowledge.
These publications warn the pilot to expect an increased takeoff distance, reduced rate of climb, and an increased true airspeed on approach and landing, although the indicated airspeed will remain the same. This can lead to floating and running out of runway options at the same time, as you will also experience a longer landing roll.
Protect yourself by checking the performance charts carefully, noting the weight of the aircraft, fine print, such as “lean mixture above 3,000 feet for maximum rpm,” as well as notes on increasing or decreasing the takeoff roll or landing roll depending on wind and runway surface condition.
Note the values given for ground roll and total to clear the mythical 50-foot obstacle at the end of the runway. The landing chart should also be reviewed, especially the notes on aircraft configuration, the effect of headwinds versus a tailwind on approach, and the increase in ground roll based on the runway surface. Dry grass, for example, increases the ground roll of a Cessna 172 by 45 percent.
If you are flying an older model aircraft with many hours on it and somewhat less than prime performance, be conservative in your calculations. Always round up for takeoff and landing distance and increase time to climb.
You may know someone who inadvertently got stuck at an airport because density altitude made it too dangerous to take off. A friend tells a story about taking his 1976 Cessna 172 to Packwood, Washington (55S), a small strip in a valley the Tatoosh Wilderness approximately 40 miles southeast of his home airport of Pierce County-Thun Field (KPLU).
Packwood airport sits at a field elevation of 1,057 feet. The runway measures 2,357 feet by 60 feet with trees and fences off the extended centerline. As he was a fairly low-time pilot, he decided the prudent thing to do would be to get one of the CFIs from the local flight school to accompany him. Both the PIC and CFI thought if they flew before noon, they wouldn’t have a problem with density altitude. They were wrong.
When they took off from KPLU (field elevation 537 feet) around 10 a.m. PT, the AWOS was reporting the density altitude as 1,300 feet. They flew around for a while, sightseeing, then headed to Packwood. They landed OK, but after three unsuccessful and wisely aborted attempts to take off, the CFI decided the best thing to do would be to leave the airplane and call his mom to come get them.
According to the aircraft’s owner, density altitude had climbed to 2,500 feet, and his Cessna 172 with the cruise prop just didn’t have the performance to get up and out safely. Later, I asked both of them if they had performed performance calculations before they took off. A sheepish look told me they had not.
The CFI replied he didn’t think it would get that bad so quickly, and the aircraft owner, calling himself PIC, deferred to him.
One client I fly with regularly takes great care with his performance calculations. He has a degree in physics, and I suspect it brings with it a primal urge to crunch numbers. As such, he carefully plans each flight, taking special care to note the aircraft performance metrics we can expect— and he explains this to me before takeoff. He aspires to be a flight instructor, so I support him in this.
The last time we flew we had the chance to listen to the airport’s recently installed AWOS, which provides density altitude. Although it wasn’t quite 10 a.m. PT yet, the temperature was approaching 80 degrees and density altitude was 800 feet. That was manageable. We intended to do laps in the pattern for proficiency.
During each lap in the pattern we checked the AWOS—density altitude kept climbing. After about an hour, we were done, more so because we were hot and our water bottles empty, although the density altitude was north of 1,500.
Personal Limitations
During the debrief, we talked about personal limitations when it comes to density altitude. There have been a few times when it was seasonally appropriate for me to put a density altitude limitation in the solo endorsement for a student pilot, but once they have their certificate, they are on their own.
This particular client informed me his personal limitation for density altitude flying out of this particular airport was 2,000 feet. That was the cutoff, he said, because he had the experience of attempting to takeoff from the runway when the density altitude was 2,000 feet with another pilot on board, and he recalled vividly that the tired flight school airplane ate up most of the runway on the takeoff roll and climb, and those power lines at the end looked a little closer than he wanted them to be.
That was not going to happen again, he said. Learning had taken place.