Wednesday, June 1, 2005
12 Tips To Beat The Heat
Here are a dozen effective suggestions for safer summertime flying
Most new-production and many high-performance aircraft have fuel-injected engines. There are some advantages of fuel injection over carburetion, but one drawback is that injected engines can be difficult to start when hot. Fuel vaporizing in fuel pumps and lines needs to be purged before the engine can fire. Here’s where a good read through the Pilot Operating Handbook (POH) is worthwhile—it should contain a hot-start procedure that takes into account the airplane’s design and make of its fuel-injection system. What is good hot-starting practice in some types can be downright damaging in others." />
5 COOL CLIMB. When the heat is up and the air density is down, it’s vital to reduce the fuel flow by a corresponding amount to get max power for takeoff and climb. There are several methods of doing this. Some engines even have an automatic mixture control to compensate for altitude, although this requires active monitoring by the pilot to make sure the system is working correctly.
The most correct method of takeoff mixture control would be to advance the power to its fullest and then lean to obtain somewhere around 75 to 100 degrees F rich of peak exhaust gas temperature (EGT)—the best-power point. Some airplanes have markings on the fuel flow gauge that provide target rates for marked altitudes, a close approximation to the EGT technique. A common practice in fixed-pitch propeller airplanes without EGT gauges is to lean until the revolution per minute drops, then enrichen for max propeller speed.
As the airplane climbs (and air density continues to drop), you’ll need to lean further in order to maintain this best-power setting. Turbocharged airplanes generally need to be full rich for engine cooling because full manifold pressure is available from the turbo, even at high DAs. Regardless of the method you use, remember that full rich usually isn’t appropriate for max performance at a high DA. Too many airplanes end up off the far end of the runway with the little red knob pushed all the way in.
6 TURBOCHARGING–NOT A CURE-ALL. One touted advantage of turbo-charging is its ability to provide power at high-DA airports. Although turbos do develop close to sea-level power at high DAs, they’re not a panacea for reduced summertime performance because:
• Turbochargers make up for the loss of ambient air by compressing induction air. This compression itself creates heat, reducing manifold pressure, so a turbo usually can’t completely compensate for high-DA power loss.
• Many turbochargers employ intercoolers to reduce the heat of compression, but intercoolers are like car radiators without a fan—they require forward motion to be effective. On takeoff, there’s not enough cooling airflow across intercoolers to make much difference. In fact, some intercooler models impede airflow enough that they provide less turbo power for takeoff than non-intercooled designs.
• Even if the turbo could provide true sea-level power, the engine still develops thrust through its propeller. In less dense, hot air, the propeller is less efficient at turning power into thrust.
• Similarly, hot air makes wings less efficient, requiring a higher true air speed to reach the same indicated air speed needed for flight.
For example, a Cessna P210 needs 2,065 feet to clear a 50-foot obstacle at sea level at an air temperature of 10 degrees C (according to the POH). Raise the temperature to 30 degrees C, still at sea level, and the obstacle clearance distance increases to 2,490 feet. The effect becomes even more pronounced at higher DAs. Turbocharging helps, but it doesn’t erase the effects of heat.
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