Hot Times

The good, bad, and ugly of hot-weather flying

July 1, 1994

Summer is the time to relax, go on vacation, or just take a few more moments to lay back and distance yourself from the slings and arrows of daily tribulation. For us pilots, those moments will certainly include flights both near and far from home base. For those of us in the northern half of the nation, the "flying season," as it's sometimes called, is full upon us now, and we'll take wing at the slightest provocation. A trip to the mountains or the shore? How about this weekend. A friend wants to see his/her house from the air? No problem.

Free of winter's always bothersome temperatures and sometimes treacherous icing and other adverse weather conditions, it can be easy to ignore the downside to summer flying. But it's there, nonetheless. Want to know what it is, in two simple words?

High temperature.

I know this shocking news comes as a complete surprise. But as accident statistics and other evidence shows, pilots do appear to sometimes lose sight of the fact that summer flying has its traps.

First of all, there's the thunderstorm problem. High temperatures set up the convective currents that make thunderstorms possible. Mix unstable air with enough heat, and you've got the recipe for convective weather.

If ever there's been a good example of the distinction between intellectual understanding and applied knowledge, the thunderstorm accident record surely provides it. We all know that flying in or near thunderstorms is deadly dangerous, yet every year some 20 or so pilots lose their — and their passengers' — lives after flying into convective weather. Even more come to grief after encounters with the gusty winds, downbursts, and other mayhem associated with thunderstorms. To stay out of trouble, apply these rules of thumb: (1) Learn all you can about the weather affecting your route of flight; (2) always have an escape plan that will allow you to fly to good VFR conditions; (3) if in doubt about the weather, or your ability to circumnavigate any thunderstorms, postpone the flight; (4) if facing convective clouds, turn around and/or land; and (5) remain visual at all times so that you can keep a minimum distance of 10 miles from any cumulus buildups.

Heat also has an adverse effect on power and performance. Simply put, heat fools engines into thinking they're higher than they really are, causing takeoff and landing distances to be longer, reducing climb rates, and lowering service and absolute ceilings. Because climb performance is a function of excess horsepower (over that required for level flight) and power is reduced by high temperatures and elevations, there may be minimal excess horsepower available for a climb, be it on takeoff or during a go-around.

Humidity further complicates things. When humidity is high, water vapor can cut engine power by up to 12 percent. It does this by displacing some of the already-sparse air entering an engine.

Because it can be so oppressive to the senses, it's tempting to think of hot air as being heavy. In fact, warm air is lighter and less dense than cold. This means that fewer air molecules are drawn into an engine's intake manifold and combustion chambers, with the consequence that the engine develops less power than it would under cooler conditions. Propeller thrust drops off, too, because low-density air provides less rearward force than the high-density air of cooler conditions. The mass of air being accelerated is much smaller.

Air density also decreases with altitude. The higher you fly, the less the atmospheric pressure and the thinner the air composition.

In the worst high-density-altitude cases — that is, situations where heat and altitude coexist, thus magnifying the power-robbing effects — several precautions can be taken to minimize performance penalties.

Because air is thinner when high density altitude prevails but the fuel being metered to the engine remains at a fixed setting, mixtures run rich when you're hot and/or high. This skewing of the air/fuel mixture further deprives an engine of its full power. To help correct this, you should lean the mixture more aggressively than usual. This reduces the amount of fuel in the air/fuel mixture and helps restore the proper ratio between the two (which is about 15:1). This precaution can make all the difference in the world in a high-altitude, hot-temperature takeoff situation, where you need all the power you can get.

Prior to a high-density-altitude takeoff, perform a full-power runup, leaning the mixture until the highest rpm (fixed-pitch-propeller airplanes) or optimum exhaust gas temperature value (usually 150 degrees rich of peak EGT) is reached. This way, you can be reasonably certain that you'll be developing all the power you can.

Another good bit of advice is to plan takeoffs so that you can take advantage of the cooler temperatures of early morning and early evening.

Don't want to monkey with these strategies? Then make sure you're flying a turbocharged aircraft. Turbocharged engines have their intake air densities increased by means of small, exhaust-driven compressors. This tricks the engine into thinking it's at a much lower density altitude than it really is and helps reverse the effects of high density altitude. But there's no free ride. Turbochargers create heat of their own, and pilots have to be careful not to exceed any temperature redlines including cylinder head, oil, and turbocharger inlet. To keep temperatures under control, intercoolers — a fancy term for radiators or heat exchangers — are used to dissipate the heat generated by turbochargers' compressed air.

What about turbine power as an antidote to heat effects? Turbines work well under high-density-altitude conditions — up to a point. Sophisticated fuel control units maintain the correct fuel flow automatically for any given ambient conditions, but heat is still a problem. To keep combustion component temperatures below redline, reduced- power takeoffs may be in order. At cruising altitudes, higher than standard temperatures make for less efficient cooling, so an entire flight in high temperatures may be conducted at lower than normal power settings.

For piston engines, high ambient air temperatures mean hotter cylinder head and oil temperatures — high enough to warrant a reduction in power, an increase in airspeed, and/or opening cowl flaps (if so equipped) to keep the needles in the green arcs. If the needles head for the red arcs, then performing all three of the above measures would definitely be in order.

For pilots flying fuel-injected airplanes, hot days concoct a special nightmare: the dreaded hot start. This problem raises its ugly head after engine shutdown, when high air temperatures and a piping-hot engine conspire to vaporize the fuel left in the fuel lines, flow divider, and other fuel-delivery components. When the hapless pilot goes to restart the engine, there can be big, ego-busting trouble. Vapor lock prevents fuel from reaching the cylinders, and the engine won't start. The best approach to avoiding this scenario is to follow the manufacturer's recommended hot start procedures. Even then, hot starts sometimes seem to require equal parts of science, art, and luck.

Pilot's operating handbooks usually recommend use of the fuel boost pump to prime the engine before attempting a hot start. The theory here is to force raw fuel through the lines and eliminate vapor that way. (Check the manual for the airplane you fly for specific procedures. They do vary.) But sometimes, it's just too hot under the cowl, and fuel revaporizes. So you crank and crank, prime and prime, and still no start. By this time, your efforts have drawn the attention of all nearby, and you probably have a flooded engine, which will require a different starting technique (usually full throttle and mixture at idle cutoff until the engine roars to life; again, check your manual).

Hot starts can be a pain, to be sure, but the summer sun doesn't confine its work to engines. It'll do a job on paint and interiors, too. Without the benefit of a hangar or other shelter, paint oxidation is speeded up by continued exposure to the sun's rays. Without sun screens in the windows, seats, glareshields, and plastic and rubber trim can dry up and crack after a few months of sitting in the boiling hot sun.

Avionics damage is another possibility. Temperatures inside an enclosed, unprotected cabin can easily reach 120 degrees Fahrenheit in the heat of the day. Even with avionics cooling fans, it'll be a long time before the cool air at altitude reaches radios and other electronic cockpit gear. For the avionics, it'll be a hot taxi, a hot runup, and a hot climb. Do this often enough, and sensitive electronic components will give up the ghost.

Heat also causes fuel to expand. Fill up in the heat of the day, and you'll take on fewer pounds of fuel. We measure fuel in gallons, but engines burn it by the pound. So a cold-temperature fill-up will let you cruise farther and fly longer than a load of fuel taken on in the middle of a hot day. It's something to remember when preparing for a long summer cross-country flight.

Last but certainly not least, heat affects pilots and passengers. After a few hours in the cockpit — especially those with large, greenhouse- like windows or canopies — dehydration sets in. Premature fatigue soon follows, exacerbated by engine noise. Then comes a series of potential physiological responses that can affect the quality of decision making and the general mood on board a long flight in the sun.

Is everything all bad when temperatures soar? Not by a long shot. Perhaps the biggest advantage of high temperature is that true airspeeds will be higher than in colder conditions, thanks to the thinner air. Batteries live longer, preheats aren't needed, you don't have to dress like an Eskimo, and an ice- or snow-covered airplane will never await your arrival at the tiedown.

The bottom line? Take advantage of this summer's flying opportunities, but anticipate less power, and take preventive steps on those scorching hot days. Winter's dark days will be back on us soon enough, so enjoy the warmth while you can.