California and the southwest are in the grip of a heat wave, and temperatures are forecast to reach 120 degrees in Phoenix today for just the fourth time in history. Of course when the temperatures soar, it's harder for planes to take off and American Airlines has already canceled over 30 flights scheduled to fly in and out of Phoenix today. But while high temperatures can be bad for the airlines, they are even worse for General Aviation pilots, who fly less capable airplanes that often lack air conditioning. The latest episode of the Aviation News Talk podcast talks in length about tips pilots can use when flying to mitigate the effects of high heat on their airplanes and theirselves. If you're viewing this with an iPhone or iPad, use this link to get to the podcast Here is a very brief summary of some of the points covered in this podcast episode.
Density Altitude
Typically, during flight training, pilots are required to study density altitude, which are the effects of nonstandard temperature and nonstandard pressure on aircraft performance. However, some pilots rarely get to encounter the kind of severe conditions we currently have. And, since pilots are not required to study anything about how hot weather affects their own performance, I give a few practical tips pilots can use to stay cool and fit while flying in hot weather.
Let’s review Density Altitude and aircraft performance. Air density is affected by changes in altitude, temperature, and humidity. Typically, most aircraft performance charts are based only on pressure altitude and temperature, and they don’t account for humidity at all. That’s probably because most charts and tables are only plotted in two dimensions, and it would be difficult to add humidity as the third dimension. However, I have found one resource on how humidity impacts aircraft performance, and I talk about it in the show.
When the weather is hot, the performance chart that most pilots are likely to check is the takeoff distance chart. That’s because pilots don’t want to run out of runway when they’re taking off! But one chart they often forget to check is the climb rate chart. Yet, that is typically the chart that gets pilots into trouble during high density altitude situations.
Most pilots do a reasonably good job of calculating a aircraft performance using temperature, however the majority of them make the same serious error when they are selecting the pressure altitude in the chart. Typically on one axis of a chart, or one part of the table, altitudes are shown and thousands of feet. And if we were going to South Lake Tahoe, which has a field elevation of about 6,200 feet, they would use the field elevation for the altitude shown on the chart. That’s because many of them either overlooked that the chart says pressure altitude, or they don’t know what pressure altitude is.
The definition of pressure altitude is simple; pressure altitude is the altitude you read on the altimeter, when you set the altimeter to 29.92. So to calculate pressure altitude, you need to know the current pressure at the airport where you are landing or taking off and the altitude of the airport. I discuss how to calculate pressure altitude from this information in this podcast episode.
Hot Weather Flying Tips
Here are some tips for flying on hot days or with high density altitude. Note that these are in no way a substitute for getting a mountain checkout from an experienced flight instructor. First, remember that you were unlikely to get the same performance as what’s in the POH performance charts. So to improve your performance, you may want to travel with less weight. Often pilots automatically fill their fuel tanks whenever they land. You should think twice about doing that if you’re taking off from a mountain airport.
On hot days, it’s also best to fly in the early morning or late afternoon. I like to be done flying by 11 AM in really hot weather. You must definitely watch your aircraft temperatures during the climb. In particular, pay attention to your cylinder head temperatures for CHT’s. Most mechanics would recommend keeping these temperatures below 400°F if possible and never to exceed 420°. Yes, many POHs say the maximum is 460°F, but your engine is probably not going to be happy operating at those kinds of temperatures.
If you do find your CHT’s rising above 400, there are three things you can do to cool the engine. The best thing is to climb at a slower rate. So if you’re climbing at 600 feet/minute, try climbing at perhaps 400 feet/minute to increase the airflow through your engine. Of course with a lower climb rate, you might not be able to out climb the terrain, so you may need to circle as you climb before turning toward your destination.
Another thing you can do is to enrichen the mixture, though most likely in a climb, you're already operating with the mixture full rich. The last thing you can do if the temperatures are rising is to open the cowl flaps, if the aircraft is equipped with them and they're not already open.
Keeping the Pilot Cool
In the podcast, I run through a number of tips for keeping the pilot cool when it's hot, and I'll share my favorite one here. For longer trips, I carry a Camelbak hydration pack full of water, and I use a trick I learned from a client when she bought a Diamond DA40 and we flew it back from Florida to California. Each morning, I fill the Camelback with as much ice as I can fit in it. Then I fill it with water. This gives me enough water for the entire day, and there’s usually still some ice left in it at the end of the day, so the water stays cold all day. You can find more of my Hot Weather Tips for General Aviation Pilots in episode 16 of the Aviation News Talk podcast. Fly Safely, and Fly Cool!
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