The Monmouth Area Flying Club

Things to Think About When Going Flying on a Hot, Humid Day

The season will shortly be here that brings hazy hot and humid conditions. That should make us think about density altitude. Warm air holds more moisture than cold air and is thus less dense than cold air. Anyone who has flown in winter and summer has noticed how the plane jumps off the ground in winter and climbs much better due to the cold dense air. In summer on a hot humid day, you notice the take off run is longer and the plane does not climb as well. The engine produces less power and the plane has to reach a greater ground speed to produce the required take off airspeed. (Wind speed considered to be still air.)

Another thing to think about is that on landing at your normal approach airspeed, the aircraft will have a higher ground speed and will require a longer roll out. There may be situations where you have to wait until the air cools towards evening if you have a short runway or obstacles to clear before you attempt a takeoff.

How do we find out the density altitude? Dial AWOS

Or use your E6B computer (you don't have to plug it in)

  1. Set the altimeter in the plane to standard pressure 29.92 and read the altimeter
  2. Read the outside air temperature on the plane's thermometer

If it is that close to the maximum performance of the aircraft be sure to get out the performance charts and do the required calculations. Keep in mind that the charts are for a new plane flown by a test pilot and figure in a fudge factor.

With these two values on your E6B set air temperature over the pressure altitude and read the density altitude at the arrow. Then go to the performance tables in the Pilot Operation Handbook and follow the directions to see how much runway you will need. You will also need to figure the weight and while you are at it, calculate the center of gravity or balance.

Also, keep in mind that as the air cools to within a few degrees of the dew point the moisture will condense and become visible such as fog, dew, frost, clouds, rain or snow. As warm air rises it reaches its dew point and forms cumulus clouds. If the air is unstable you will see vertical development (towering cumulus). If these clouds continue to develop they could turn into thunderstorms (cumulonimbus).

On a 90 degree day and a density altitude of about 2000 feet from the Piper Arrow charts you can expect a ground run of about 1300 feet with no flaps. From the chart with the density altitude of about 2000 feet you might expect and initial rate of climb of about 800 feet per minute. In one minute you will cover 1.6 miles at 96 miles per hours (best angle speed) and no wind. (How far away and how high are those wires?) And don't forget this is based on a new airplane flown by a test pilot.

Remember the old saying "I'd rather be down here wishing I were up there than up there wishing I were down here".

Frank Fine - MAFC Chief Flight Instructor