It was late summer, and I nursed the old Bellanca Cruisemaster higher as we passed over Blue Mesa Reservoir near Gunnison, Colo. The engine was leaned for our 9,500-foot altitude, but looking straight ahead at the crest of the Rockies, it was obvious I still had higher to climb.
The Continental Divide cut directly across our path 30 miles ahead. Monarch Pass reared up as I followed U.S. Highway 50 east into the tall rocks, leaning the mixture a little more and hoping the 190 hp Lycoming could find extra power to somehow lift us above the jagged rocks of the pass at 11,300 feet.
I had passed this way probably a dozen times before in a variety of airplanes, from Super Cubs and Decathlons to Scouts and Skyhawks, and I was grateful that it was a clear day with little wind and hardly any turbulence. I knew it was unusual to see an OAT of 25 degrees C, warm for the high Rockies in late July.
I had already crunched the numbers through my trusty E6B, and the density altitude at 9,500 feet pressure altitude worked out to a discouraging 12,700 feet. The crest of the pass was almost another 2,000 feet above our current height, so we’d probably need to be at a theoretical 14,500-foot density to squeak across the summit with a few hundred feet to spare.
Perhaps in anticlimax, we picked up a little lift as we lofted uphill toward the high mountains. We soared over the final high-way switchback to the viewpoint on the Continental Divide, peppered with cars and tourists, many probably startled by our 500-foot pass above them. Then, we dropped over the eastern rim and headed toward Montrose and the Midwest.
Understanding Density Altitude
High density altitude isn’t much of a concern over most of the U.S., but it becomes a problem each summer in the Southwest. The simple increase in temperature by 15 or 20 degrees C in the middle of the year thins the air and makes life more difficult for pilots of general aviation airplanes when the days grow long.
One of the reasons pilots get into trouble in summer is that they don’t fully understand the effect of the temperature change. Reading the OAT and translating that through the performance charts often doesn’t even come close to predicting airplane performance.
High density altitude becomes a problem each summer in the southwestern U.S. Grand Canyon National Park Airport (KGCN) sits at an elevation of 6,609 feet, but density altitude reaches much higher during the hot summer months.
flight to the U.S. after delivering a new airplane in late January. As it happened, I knew the captain of the flight, and he asked if I wanted to ride jump seat for part of the trip. (This was obviously well before 9/11/01.)
I boarded with the crew, stowed my gear and climbed up to the cockpit. The captain was out attending to last-minute details, so the first officer and I discussed the flight ahead. Elevation at Johannesburg is just over a mile above the sea, and Jan Smuts Airport has one runway that stretches nearly 14,500 feet. Our departure was scheduled for midnight, but the temperature was still almost 30 C as we prepared for pushback.
I asked the FO about our takeoff roll, he shuffled through some papers and finally found the number. “It’ll be about 13,800 feet,” he said casually.
I was stunned. It had never occurred to me that anyone could predict performance that accurately on a 400-ton airliner. Sure enough, a half-hour later when we rolled down the runway, I was amazed at the accuracy of the copilot’s calculation. When he called “Rotate” and the captain finally lifted off, the REILs were directly ahead, probably less than 700 feet away. We roared above the lights at 50 feet AGL and climbed away toward London.
Fortunately, we don’t have the same problems in personal aircraft, but figuring accurate density altitude can still be a challenge.
Part of the problem is that pilots sometimes use the wrong temperature to calculate DA. Most of the time, we fly from smooth, dry runways, typically constructed of concrete, asphalt or a combination of the two. When the outside air temperature soars above 38 degrees C (100 degrees F), runways tend to absorb heat and bake in the sun, and the actual runway temperature may be far warmer than the reported OAT.
On a very hot day, consider doing part of your checklist before engine start to save time and avoid unnecessary engine temp increase.
In the true dog days of summer, airports at places such as Phoenix, Tucson, Las Vegas, El Paso, Dallas and other hot spots may announce the runway temperature on the ATIS (“Temperature 39, runway temperature 48 degrees), making density altitude calculation easier. Sometimes, ATIS will even include a computer-generated density-altitude computation. If they don’t, the higher runway temp is the one to use for density calculation.
But, wait. There’s another factor that should be considered. Tightly cowled engines operate on induction air that may be considerably hotter than even the runway temperature. Induction tubes and intake manifolds heat up disproportionately as the engine warms, heating air trapped in the engine compartment and contributing to a further escalation of temperature inside the cowling. If you’re operating from a major terminal such as those mentioned above, you may be forced to wait in the run-up area for several minutes. Meanwhile, the induction temperatures may be climbing well above any reasonable limit.
There are ways to shortstop this problem if you’re operating from an uncontrolled strip or an airport that will allow you to depart on pretty much your own schedule. If you use the standard C-I-G-A-R-S-R-LCA checklist or some variation thereof, you can shorten the run-up time without compromising safety.
You can accomplish several of the checklist items before even starting the engine. C (controls) doesn’t rely on engine operation, G (gas) and A (attitude) can also be set in advance. I (instruments) obviously demands that the engine be running, as does R (run-up) but you can expedite even those. You can do the instrument check during taxi, and some pilots who operate in hot climates make it a habit of performing the run-up for the next flight immediately after landing from the last one. Security, Radios and Lights/Camera/ Action need to be completed immediately before taking the runway. Many mechanics suggest the engine should be ready for full power application any time the oil temp is above 100 degrees F. In summer at the places above, that will be practically all the time.
If you’re uncomfortable with doing the next flight’s run-up after the last flight, consider performing the run-up as you take the runway; then, push power on up for takeoff. If it’s necessary to hold on a hot ramp, face the airplane into whatever wind there is and make certain cowl flaps are full open.
When you do get off the ground, consider a cruise climb speed 10-15 knots above Vy after you clear the first 500 feet. This will pump more air through the cowling and should help cool the cylinders. It will also provide a better view of other traffic, more than a peripheral benefit.
Another hedge to keep both the airplane and the passengers happy is to flight-plan your trips as early in the day as possible, before the hot Earth begins to radiate heat uphill.
A second precaution to counter thermal turbulence is to cruise at a higher altitude, possibly above most of the chop. The obvious benefit is a smoother ride, but another advantage is the pad of altitude between you and the ground.
If you should encounter strong turbulence, especially significant up- and downdrafts, remember not to try to maintain altitude. If you’re VFR, let the airplane ride with the thermals (consistent with terrain clearance). When filed IFR, you’ll need to maintain a consistent altitude, but if that’s not possible, advise the controller, and again, remember to fly the airplane first, maintain a precise altitude second.
If things become unmanageable, slow the airplane to maneuvering speed (Va) to minimize the chance of structural damage. If you can’t remember Va for your make and model, you can estimate it by multiplying the flaps-up stall speed times 1.5.
When you arrive at your destination, don’t worry about padding your approach speed unless you’re concerned about wind shear. Use the same indicated airspeed, as it automatically compensates for density altitude.
A few other tips can make life easier and safer in the summertime. Remember that any major temperature variation from a standard 70 degrees, either on the high or low side, may incline us to skip items on the preflight. That’s exactly the time when we need to be most cautious.
If you’re travelling cross country on a multi-day trip, consider waiting until just before departure to fill the tanks rather than doing it the night before. That way, you can taxi out without overboarding fuel thru the fuel vents or out the filler caps.
Buy a good set of interior sun shields for your airplane if you need to leave it outside overnight in a hot climate. This can lower interior temperatures by 20-40 degrees C to make life easier for avionics, controls and the glue in your upholstery. It also keeps prying eyes from evaluating your avionics.
If you don’t have sun shields, be sure to stow headsets out of the sun so they won’t be too hot to handle, and cover the yokes with a towel for the same reason. Carry plenty of water on any trip away from settled areas, especially in the desert. You can survive for quite a while without food but only three days without water.
Then, there’s the weather. The dominant phenomenon of summer is one of the worst in the sky: thunderstorms. These aren’t specific to summer, but they’re more common in the warm months. Entire books have been written on thunderstorms and how to fly them, so we won’t attempt to paraphrase them here. The best advice is probably the simplest.
Don’t. A few airplanes, mostly Concordes, U-2s and SR-71s, can top thunderstorms, but most other aircraft can’t even come close. If you must operate in the vicinity of CB activity, try to stay at least 30 miles away from it and never, NEVER fly under the anvil. That’s where the hail lives.
Also, remember that rotation around any extreme low-pressure system, such as a thunderstorm, is always counterclockwise (in the Northern Hemisphere). If you plan to circumnavigate one of the monsters, fly the right side where you’re more likely to find tailwinds.
Summer flying isn’t really that special. If winter has more clouds, colder temperatures and occasional no-go conditions, summer is more clement, known for shirt-sleeve weather, often friendlier to pilots and airplanes.
Just be sure to watch for airplanes coming through Monarch Pass.