Flying Seasons

Density Altitude

July 1, 2007

It isn't just for mountains anymore

Here's what the corporate world calls a "takeaway," right at the top of this article — flatlander pilots should lean to maximum rpm prior to takeoff whenever the departure airport's density altitude is at or above 5,000 feet. You'll find that recommendation in a Lycoming service instruction. (Pilots of turbocharged aircraft must always take off with the mixture full rich.)

AOPA's headquarters airport, Frederick Municipal Airport in Maryland (elevation 303 feet), would have to experience a 135-degree Fahrenheit day in order to have a 5,000-foot density altitude (assuming pressure altitude is the same as airport elevation), but at Boulder Municipal Airport in Colorado (elevation 5,288 feet), that's the density altitude on nearly freezing day. On a 100-degree F day, Frederick reaches a density altitude of more than 3,000 feet — enough to warrant a pilot's attention — but Boulder's density altitude soars to nearly 9,000 feet.

Accident reports in the AOPA Air Safety Foundation database indicate that the biggest density-altitude related problems occur when flatlander pilots operate at a higher-elevation airport, but fill their tanks, load their airplanes in excess of maximum gross weight, leave the mixture full rich for takeoff (as they were taught at lower elevation airports), and operate in what they think are pleasant 70- or 80-degree F mountain temperatures. But those can be killer temperatures.

A case pointed out by Boulder flight instructor Elliot Crawford involved a 70-degree F day at Lake County Airport in Leadville, Colorado (elevation 9,927 feet). A Mooney pilot was seriously injured when his aircraft slid off the end of the runway and continued 422 feet down a steep slope. He had tried to climb at too steep of an angle (bad decision) after accelerating in ground effect to best-rate-of-climb speed (good decision), lost 20 knots of airspeed, and settled back to the runway. Density altitude was between 12,500 and 13,000 feet, according to the accident report.

Crawford tells of another pilot who had a happier outcome to his Colorado flying experience. A Cirrus SR20 pilot departed Erie Municipal Airport (elevation 5,130 feet) but immediately returned, saying there was something wrong with his engine. A flight instructor got aboard and found that the engine was performing normally for the density altitude that day and would have done better if only the pilot had leaned properly — or at all.

Aside from leaning, here are some other tips from Crawford, who operates in Boulder density altitudes that top 5,000 feet 11 months of the year according to yearly temperature averages.

  • Trust your airspeed indicator, not your eyes, when landing. Groundspeeds can be up to 20 knots faster than you are used to when using the same indicated airspeed (IAS) required by the pilot operating handbook.
  • Fly in the evening or early in the morning when temperatures are lower.
  • Call a local instructor at the airport where you are going.
  • Before flying to a high-altitude airport, know whether your aircraft climbs more efficiently with the first increment of flaps. Many aircraft do, but your results may vary and that first notch may add more drag than lift.
  • Be sure the aircraft's weight is below 90 percent of maximum gross weight.
  • Don't fill the tanks (see previous tip).
  • Fly shorter legs and make extra fuel stops (tough suggestion to accept, but it results in less exciting takeoffs).
  • Be ready to ferry one passenger to a lower density altitude, then come back for the other. If you are unsure of conditions, fly around the pattern once alone without baggage to test your aircraft's performance.
  • Have 80 percent of your takeoff speed at the runway's halfway point, or abort. For Crawford, that means having 48 knots IAS in a Cessna 172 at the halfway point.

Density altitude worries aren't just for Colorado pilots anymore. When the temperature is 105 degrees F on a windless day, the airport density altitude is nearly 3,000 feet, and your Piper Archer (flaps up) is loaded to the gills as it taxies towards a 3,000-foot runway with a 50-feet tree at the end, have second thoughts. Even third thoughts. Because your takeoff distance to clear a 50-foot obstacle under those conditions is 3,200 feet.

E-mail the author at [email protected].

Al Marsh

Alton K. Marsh | AOPA Pilot Senior Editor, AOPA

AOPA Pilot Senior Editor Alton Marsh has been a pilot since 1970 and has an airline transport pilot certificate and instrument and multiengine flight instructor certificates, aerobatic training, and a commercial seaplane certificate.