By ED BROTAK
With many people vacationing, summer is typically a busy time for aviation.
Typically, flying weather is better in summer — if you avoid the thunderstorms I talked about in my last article. There are fewer “big” storms and fronts with widespread clouds, precipitation, and strong winds.
But summer also brings a different set of weather concerns.
We talk about the summer heat, but how hot is hot? Well, even Fairbanks, Alaska, has hit 90°.
“Heat waves” with stretches in the 90s are common throughout much of the US and into Canada. High temperatures of 100° have occurred in many locations and are common in the Desert Southwest. There, highs can soar to 110° or beyond.
One of the most serious problems that comes with hot weather deals with the effects of temperature on air density. Air expands when it is heated and its density decreases accordingly.
Thinner air causes a number of problems for aircraft operations. Thinner air means less oxygen for the combustion process in the engine and a reduction in power. There is less “pulling power” by the propeller or propellers.
And, most importantly, there is less lift produced by the thinner air flowing over the wings and less lift produced by the blades of a helicopter.
This problem is compounded at higher elevations. Air density decreases exponentially with height. This effect is constant and can be allowed for. But add in temperature fluctuations, and the situation becomes more complex.
To help pilots determine the combined effects of elevation and temperature, we have “density altitude.” For example, at an actual height of 5,000 feet if the air temperature is 90°, the effective altitude or density altitude is 8,200 feet. This would mean a significant decrease in air density.
This dangerous combination of elevation and heat is called “High Density Altitude” or HDA. This is a rather misleading title since it is the low density of air at high altitudes that causes the problem.Seldom is this a problem at low elevations in summer regardless of temperature. The air is dense enough to start with to minimize the effects of heating. Even flying high above these locations is safe since the temperature has decreased quickly as you move away from the hot ground below.
But at higher elevations, the surface — the ground itself — is higher. The Phoenix Sky Harbor International Airport is at 1,135 feet Above Sea Level (ASL), but with summer temperatures often above 100°F and sometimes up to 120°F, HDA can be a problem. Denver International Airport at an elevation of 5,431 feet ASL has hit 101°F.
And keep in mind, air temperatures well above the ground would be considerably warmer than “normally” expected.
Check your aircraft’s performance charts on operating in “high and hot” conditions.
There’s also the Koch Chart, which provides estimates of the decrease in the rate of climb for different altitudes and temperatures.
Certainly, a reduction in load is a standard recommendation. In HDA conditions, takeoff distance is greater and the climb rate slower. Obstacles — natural and man-made — that are usually cleared easily can now become real threats.
Even if takeoff is successful, loss of lift can occur later in the flight just at slightly higher altitudes.
For landings, you’re coming in faster, so you’ll need more runway to allow for the greater roll distance.
Another problem that arises in summer is widespread turbulence, typically not severe, but enough to produce a “bumpy ride.”
The turbulence is produced by localized convection, “thermals” as they are often called.
Heating of the earth’s surface by the strong summer sun can result in “super-heated” air just above the ground. With steep lapse rates at low levels, this hot, less dense air will rise quickly until it reaches thermal equilibrium with its surroundings.
Often the air isn’t rising continuously, but rather in bursts or bubbles. This produces the vertical bouncing effect.
Cumulus clouds are a tell-tale sign of such convection, but if the air lacks moisture, dry thermals can still exist.
If you want a smoother ride, plan your flight when the sun isn’t that strong — early morning or the evening. Flying higher, over the low-level turbulence, is also possible if that is an option.
Heat can affect an aircraft’s mechanical functions. Certainly, engine overheating is a major concern. Be sure to monitor cylinder-head temperatures and oil temperature regularly.
The FAA recommends to lessen the risk of engine overheating you can increase airspeed, enrich the fuel-air mixture, and/or reduce power.
Of course, heat can also affect a pilot. Most importantly, heat can impair mental functions, including decision making.
Considering that the body cools itself primarily be evaporating sweat, the relative humidity is also important, as well as the actual temperature. High humidity lessens the evaporative cooling effect. The widely used Heat Index combines the effects of temperature and humidity to produce an “effective temperature,” one that the body actually feels.
Dry air would promote more cooling evaporation, but then dehydration becomes a concern. Also keep in mind, the cockpit temperature and humidity may be significantly different from outside conditions. Proper hydration is a must to avoid heat-related problems.
Another problem that arises in the summer in some areas like the Southwest is dust storms. Strong winds can easily pick up smalls grains of dust or sand. They can be ingested into an engine, damaging it or even shutting it down.
Loss of visibility is a major concern. “Brownout” conditions can drop visibility to near zero in a matter of minutes.
And even in these dry climates, thunderstorms can develop in the summer. But often whatever rain that is produced evaporates in the dry air below the cloud base. But the strong downdraft winds can hit the ground, producing “Haboobs” as they are called here.
Such storms can appear as a wall of dust, which can be a mile high and over 100 miles wide and can move at speeds up to 40 mph. The National Weather Service would issue either a “blowing dust advisory” or “dust storm warning” if conditions warrant.
And finally, especially out west, summer brings the wildfire threat. The areas near and over a fire are typically restricted due to aerial firefighting operations.
The flying weather there isn’t good anyway with strong winds and convective columns that can rise tens of thousands of feet. Restricted visibility due to smoke can be a problem many miles downwind.
Whatever the weather, remember to fly safe.
Dr. Ed Brotak is a retired meteorology professor and now a freelance writer. He has written extensively on aviation safety issues including weather hazards and the bird/wildlife strike problem.