1 HOT STARTS. 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.
When do you need to perform a hot start? If the oil temperature is in the green arc when you turn on the battery, you’ll likely need a hot-style start to fire up. This is most common when you’re making a quick turn for fuel or to drop off a passenger, even more so if the airplane sits for 15 or 20 minutes, enough time to completely heat soak and vaporize fuel. Shutting down and immediately restarting is sometimes less difficult than starting after a short delay. But I’ve seen green arc oil temperatures before the first flight of the day, when the airplane has been sitting out in the summer sun.
To reduce the level of heat that builds up during a quick turn:
• Park in the shade if possible.
• Park directly into the wind—air will flow into the cowling.
• Leave cowl flaps open to ventilate the engine compartment.
• Open the oil access door and, if the design allows, open the cowlings. (Don’t do this in windy conditions, or if there’s a change, another airplane’s blast will hit the open doors.)
• When conditions dictate a hot start, do the hot-start procedure first. Trying the normal procedure first may result in not a hot and flooded engine.
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.
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.
7 CARBURETOR ICE. Summertime is not only the hottest part of the year in many parts of the country, but it may also be the most humid. Carbureted engines reduce internal temperatures by as much as 50 degrees F, making engine-choking ice a possibility in moist air even during summertime temperatures. When the relative humidity exceeds about 50% and/or the dew point is within around five degrees of the temperature, do the following:
• Use full carb heat at low power settings and as otherwise recommended by the POH.
• Clear the engine by advancing the power slightly every 30 seconds or so if you’re in an extended glide (such as a simulated engine failure or a long, power-off letdown).
• Consider installing and occasionally calibrating a carburetor temperature gauge and using carb heat as necessary to keep temperatures in the induction system above freezing.
8 AIRFRAME ICE. If you’re flying a turbocharged airplane at high altitude, don’t forget that the temperature aloft may be cool enough to support icing, even when the surface is a very hot furnace. Add pitot heat and other ice-protection systems checks to your checklist when you’re climbing to altitude, even on the hottest days.
9 CONVECTIVE TURBULENCE. Hot surface air sets up flows that can cause moderate or greater turbulence close to the ground. It’s generally accepted that noon is the time to end all summer flights in the desert Southwest because turbulence later in the day will be intense. Hot days over less arid climes still may produce uncomfortable-to-dangerous convective turbulence close to the ground. Conditions get even worse around mountains or when surface winds are high. Avoid the worst by:
• Planning flights for the morning hours or in the early evening.
• Cruising 5,000 feet or higher above terrain on hot days.
• Planning descent to spend as little time as possible at low altitudes.
• Avoiding areas with visual or weather briefing reports of dust devils or blowing dust.
10 THUNDERSTORMS AND SEVERE WEATHER. Frontal thunderstorms are pretty easy to anticipate with the scantest of preflight weather briefings. What’s much harder to predict, however, are the air-mass thunderstorms, those that crop up from the heat of the day away from surface fronts. Convection makes the atmosphere like a pot of water just about to boil—the potential is there, but you can’t tell all hell is about to let loose until it begins. Here are some telltale signs of air-mass thunderstorms and severe weather:
• Humidity above roughly 50% and/or a temperature and dew-point spread within 5 degrees F.
• Winds aloft blowing from the direction of a body of water, adding moisture to the air.
• A high-pressure system with colder-than-standard air aloft.
• Indication of a strong jetstream above your route of flight.
• Winds blowing upslope, especially in mountains.
• Unstable air, indicated by a negative number on the Lifted Index (ask your weather briefer).
• Widespread pilot reports of turbulence and/or building cumulus clouds.
• PIREPs or METARs indicating hail of any size.
• Local media reports of expected severe weather.
Just as with turbulence, air-mass thunderstorms tend to build with the heat of the afternoon. If conditions favor storm development, consider sitting out the hottest part of the day, flying instead in the morning as well as the early evening.
11 LEANING FOR LANDING. As the airplane descends, air density increases, so you’ll need to enrichen the mixture to prevent the engine from getting so lean it stumbles or quits. Ideally the mixture should be gradually returned to the full-rich position during the descent, so full power is available should you need to abort the landing. But what if you’re landing at a high-DA airport? In that case, the justification for full rich goes away. Instead, you need to aim for a mixture control position roughly appropriate to a best-power setting at full throttle for that possible go-around.
If you routinely fly in high-DA conditions, start looking at the physical position of the mixture control for takeoff in those conditions and return the control to that approximate position prior to landing. If you’re flying into an airport at a higher DA than you’re used to, you might work the mixture control to one-half to one inch away from fully forward as an initial landing position. Knowing this isn’t scientific, your go-around procedure would include advancing the throttle, establishing a climb attitude and then adjusting the mixture as necessary to hit your fuel flow, revolutions per minute or EGT target for your present altitude.
12 FUEL VENTING. Have you ever seen fuel dripping from a Cessna 172’s wing vent on a hot summer’s day? Aviation fuel expands with an increase in temperature. In some airplanes, the expansion may be huge enough that fuel begins to siphon through fuel tank vent lines. Don’t fill the fuel tanks completely to the brim if the airplane is going to sit in the heat for some time before flying. Remember, the fuel you have put in the tank may already be hot and expanded—meaning there’s less time in the tanks than it would appear. You may decide to wait until shortly before takeoff to top off the tanks in the hot season, although if you have to taxi to the fuel pump, you may be setting yourself up for a hot start after fueling.