Wednesday, June 1, 2005
12 Tips To Beat The Heat
Here are a dozen effective suggestions for safer summertime flying
2 GROUND LEANING. Most aircraft fuel systems are set up to run extremely rich at idle power. This is a result of setting fuel flows for engine cooling during extended climbs. On a hot day, when ambient air is less dense, the mixture may be so rich that the engine runs rough and carbon begins to build on spark plugs. This will, in turn, reduce power for takeoff. Prevent carbon buildup by manually leaning the mixture during ground operations. The percentage of power is so low during taxi that leaning won’t cause internal cylinder temperatures and pressures to lean up to the point where the engine quits. Hold short of that extreme, and you’ll keep the plugs clean for takeoff. In fact, some experts suggest routinely running so lean on the ground that the engine can’t develop full power when the throttle is moved forward—this is to remind the pilot to enrichen the mixture sufficiently for takeoff.
3 DENSITY ALTITUDE. Density altitude (DA) is pressure altitude (close to altitude above sea level) corrected for nonstandard temperature. A high DA results when hot air makes the density of the air less than what would normally be at a given pressure altitude. A look at aircraft performance charts shows that a noticeable reduction in performance begins to show at DAs as low as about 3,000 feet. That means that an airport at about 1,500 feet MSL becomes a high-DA airport when the outside temperature exceeds about 85 degrees F—which perfectly describes the Midwest on a typical summer afternoon. Even low-altitude airports begin to exhibit high-DA traits during the hottest of summer days. And, of course, higher elevation fields can be high DA almost all year long. Humidity displaces oxygen, so high humidity can negatively affect aircraft performance on a hot day, too.
Use aircraft performance calculations conservatively because they assume a new airplane, a new engine, a perfect runway as well as an exemplary pilot technique. They also assume that the measured temperature is correct. Have you ever stood on an asphalt parking lot during a hot summer day? The local temperature may be significantly higher than ambient air nearby as the dark surface absorbs heat and transfers it to the air. DA on the runway, where it matters most, may be even higher than the calculated DA using the official airport temperature. Give yourself a significant margin when applying results from performance calculations.
4 AIRCRAFT WEIGHT. An airplane needs to generate enough lift to overcome its weight in order to take off. The heavier the airplane, the more lift it must create, meaning it needs additional power and airflow across its wings. Conversely, the lighter the airplane, the less power and lift it needs to fly. The lighter you can load your airplane, then, the better performance you’ll have in the summer.
Let’s look at an airplane considered to be a pretty good load-hauler, the Beech A36 Bonanza. At max takeoff weight on a 30-degree C day and a 4,000-foot density altitude, it takes about 1,900 feet to get airborne (in zero wind), and 4,000 feet to clear a 50-foot obstacle. Reduce the load by only 100 pounds, and the takeoff roll drops to 1,700 feet, with a much-reduced 3,200 feet needed to clear that obstacle. Losing weight can make a significant difference. (At least that’s what my doctor says!)
Under the same environmental conditions, the pilot of a light twin, like the Piper Seminole, can’t even climb with an engine failure on takeoff at max weight—at max gross, the PA44 nets about 50 fpm descent with a dead engine, the prop feathered and the gear up. The Seminole pilot needs to reduce aircraft weight by 200 pounds just to hold altitude on one engine on this hot day, and to realize the minimum 200 fpm single-engine climb capability most would expect from a light twin, the pilot would have to be a full 1,000 pounds below max gross weight.
This brings up another multi-engine consideration for summertime flying. Most naturally aspirated light twins have a single-engine service ceiling of around 5,000 feet at max weight. Remember that this is a 5,000-foot DA. Lose an engine at high weight, and the airplane will drift down to this DA which, in high terrain in the summertime, means an extended glide to a landing on one engine. Keep the airplane weight down, and choose your route over the lowest terrain, even in a twin.
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