Dealing With Density

Performance takes a pounding with summer's heat

June 1, 1997

It's as much a part of summer as barbecues, suntan lotion, and thunderstorms — heat and, as a result, high density altitude. Flying under these conditions requires more than a cursory glance at the aircraft flight manual and an extra glass of iced tea before departure. Even for airplanes with normally strong runway and climb performance, density altitude takes its toll.

Why? There are several factors at work here. A dramatic reduction in air mass density — factors including high altitude, temperature, and humidity, combined with low barometric pressure — deprives wings and propellers of lift and efficiency and also serves to wither the output of nonturbo engines. Because the wing is essentially flying according to indicated airspeed, high density altitude takeoffs will result in much higher true airspeeds for rotation and climbout, with a correspondingly greater consumption of runway length and a shallower climb gradient.

You don't need to be flying out of Leadville, Colorado — the nation's highest-elevation airport, at 9,927 feet msl — to appreciate the effects of density altitude. Take an airport at 2,000 feet msl, add to that an ambient temperature of 86 degrees Fahrenheit (30 Celsius) and at standard pressure and humidity, you've got a density altitude of 4,100 feet.

For illustration, let's assume that you're flying a Cessna 172P — 160 horsepower, fixed-pitch prop, weighing in at 2,400 pounds for takeoff. At sea level and 59 degrees F, you'd need about 890 feet for the takeoff roll and 1,630 feet to clear a 50-foot obstacle; once you are airborne, your climb rate is supposed to be 700 fpm at 76 knots indicated. Now perform the takeoff from the 2,000-foot msl airport at 86 degrees F. You'd need 1,200 feet to get off the runway and 2,220 feet to clear the 50-foot obstacle; climb rate would be just more than 500 fpm. The anticipated reduction in performance is about 25 percent — and is likely to be more significant in the real world of high-time airframes, anemic engines, and oft-filed propellers. Moreover, these takeoff numbers come under the category of short-field procedures — take a few extra seconds to rotate the airplane or forget to put out the 10 degrees of flap as per the book and you may use far more runway than the book predicts. Additionally, at the recommended 74-knot climb speed, your true airspeed would be 79 knots, which will have an impact on your climb gradient, potentially a problem if there's hilly terrain around.

Drag that same Skyhawk to Colorado Springs on a summer afternoon and you'll be truly amazed at its lackluster performance. At the airport's elevation of 6,184 feet, even a comfortable 70-degree F day will give a density altitude of more than 8,200 feet. Your Skyhawk will do well to get off the runway in 1,700 feet and clear the obstacle by 3,400 feet; climb rate should be a patience-straining 330 fpm. Again, these are handbook numbers that are likely to be quite optimistic in practice.

High-performance, normally aspirated airplanes reflect similar reductions in runway and climb capability with density altitude. Minimum climb-performance criteria in aircraft certification most often dictate the maximum gross weight of any aircraft with a given amount of power; as a result, by percentage, even high-powered nonturbo airplanes suffer performance degradation at high density altitude.

If the handbook numbers don't reflect realistic performance, what do you do? A conservative rule of thumb says to double the runway and obstacle-clearance numbers from the book. Such a wide margin is often suggested not so much because the handbooks are fiction — though some of the early manuals are quite optimistic — but because there are often complicating factors. For instance, at a high-altitude airport on a hot day, you'll be contending with the density altitude, as well as up- and downdrafts, turbulence, and takeoff speeds that appear to be excessively fast. (Remember, your groundspeed is your true airspeed prior to rotation, and at density altitude of 8,000 feet, a 70-knot rotation will, in fact, be almost 80 knots.) The reduced climb rate often causes pilots to hike the nose up in hopes of improving the rate; most of the time, this only squanders what little climb performance there is to be had.

Aside from shelling out the cash to buy a big, turbocharged airplane — and even that's not a density-altitude panacea — what can you do? First, consider some of these common-sense steps to mitigate the effects of density altitude.

Fly early in the day. A departure just a few hours earlier can mean the difference between getting off the runway and clear of obstacles with room to spare and an agonizingly slow, sickly ride to altitude. Besides, the ride in mountainous areas in the morning is far preferable to the belt-stretching roller coaster you'll endure in late afternoon.

Fly light. Don't take on full fuel if you don't need it. (But, naturally, maintain legal and adequate reserves.) Pack light and consider ferrying passengers into and out of really high-altitude strips rather than taking them en masse. For example, by cutting the Skyhawk's takeoff weight by 20 percent, you'll realize a 36-percent improvement in runway performance. That's a pretty good payback.

Manage the mixture. At high density altitude, your engine will produce much less than full power at takeoff, so you'll need to set the mixture accordingly. For density altitudes above 3,000 feet, lean the mixture to peak rpm for fixed-pitch prop installations. At 3,000 feet density altitude, our Skyhawk example will be doing well to see 2,400 rpm, roughly the equivalent of 70 percent power. According to Lycoming, you can lean at any setting of 75 percent or less, so you will be doing no harm — and in fact lots of good — to lean properly for takeoff. Airplanes with constant-speed props should be leaned to best-power mixture, between 125 degrees F and 75 degrees F rich of peak exhaust-gas temperature (EGT). You can find this point ahead of time by noting the EGT peak in cruise for a particular altitude. This won't be an exact reference, but because the power curve is quite flat on the rich side of peak EGT, getting in the ballpark will suffice.

Turbocharged airplanes perform better at high density altitudes because the engines are capable of sea-level (or near sea-level) power. But, as one longtime mountain flyer put it, you can't turbocharge the wings. A turbocharged Socata Trinidad, for example, will use 25 percent more runway on a day with an 8,000-foot density altitude than at sea level. Fortunately, its climb performance doesn't seriously degrade until reaching 20,000 feet. And as with the Skyhawk, the TB 21's runway and initial climb performance improve greatly at lighter weights. For a 15-percent takeoff-weight reduction, the airplane's ground-roll and obstacle-clearance numbers improve by 30 percent.

Book numbers and rules of thumb are one thing. But by far the best tactic is to proceed into high density-altitude situations gradually. Be very conservative until you have determined that your airplane will perform as advertised. Increase the takeoff weight in increments; don't just assume that because it leapt off the runway at a light loading, it will be enthusiastic at maximum gross weight. Then, with this practical experience in hand, you can relax and take in the other joys of summer.