Summertime performance considerations

Summer is here again and with it comes changes in aircraft performance. Regardless of aircraft type, performance takes a nosedive on those hot and humid summer days.

Pilots flying turbocharged aircraft are also affected but to a lesser extent. Turbocharging may mimic low altitude performance even though the density altitude is high. For the rest of us, we take whatever performance we can get! This month we review some of the performance aspects of summer flying.

Temperature would appear to have the main effect on aircraft performance from day-to-day, but to consider it separately from altitude and aircraft weight creates a misleading picture at best for piston aircraft. One important fact is performance is rarely linear when considering: changes in temperature when weight is constant, changes in weight at a constant temperature, or even changes in altitude and temperature at a constant weight. However, some basic relationships exist for most light aircraft that are close enough to linear for us to generalize.

First, as we should all be aware, as weight increases… performance decreases. How much it decreases depends on two additional variables: if the altitude has changed, and if the temperature has changed.

For example, most pilots operate their aircraft well below gross weight and so each 10-degree temperature increase has little impact relative to the previous performance figure. Load the aircraft up to max gross weight and this relationship usually changes. The same temperature increase for an aircraft at max gross will have a greater impact on performance.

An increase in altitude will increase the decrement in performance already lost to a higher temperature and it is common to see performance figures almost twice their sea-level value at 10,000 feet MSL. The change per 1,000 feet to 10,000 feet is not exactly linear but it is usually close enough to allow for good estimates for intermediate altitudes.

Humidity is a factor few pilots consider when calculating performance, and for good reason; no one provides charts for factoring in humidity because of its relatively minor impact. However, if you think you’re going to get the charted performance on a day that is significantly more humid than standard (which is 0 relative humidity in the International Standard Atmosphere) then you are fooling yourself.

So, what does one do to get a truer picture of performance under high humidity conditions? Some high-tech chart for converting altitude for humidity effects probably exists somewhere in the world, but I use an easy method that is probably an equally good approximation.

If you have ever heard of the “heat index” then you can probably tell where I am going with this. The heat index calculates the impact of humidity (by using the dewpoint) on temperature to produce a temperature value that more accurately describes how our body will react. Some charts will readily figure the heat-index if you know the relative humidity.

A higher dewpoint means a higher heat-index value. I use this index value as the actual ambient temperature in my summertime performance calculations. Considering performance charts as they are, this is a conservative approach to figuring performance.

Of course, all performance calculations really boil down to the effects of varying density altitude. As a refresher, recall that density altitude is true altitude adjusted for nonstandard temperature and pressure. As temperature increases and pressure decreases, the density altitude increases; meaning the aircraft will perform as if it was higher than it actually is.

Sometimes only temperature or pressure alone is responsible for the change in density altitude, and sometimes a low temperature cancels out a low-pressure effect such that density altitude is the same as true altitude.

Unless you are required to figure density altitude before using your performance charts, you are not likely to directly notice the impact of either temperature or pressure on the final performance value, but rest assured, they both play an important role.

I like to figure density altitude separately from the rest of the performance calculations just so I will know ahead of time what kind of impact to expect.

Density altitude is affected by humidity but about all we can say for sure is high humidity increases density altitude, thus decreasing performance. The reason for humidity’s impact on performance is clear. Moisture displaces air molecules in a given volume of air, making that volume less dense in terms of the amount of air present.

Moreover, since our aircraft engines do not run so well on moisture, that’s a bad thing! The moisture level is never high enough to “kill” the engine but power output will be lower under high moisture (humid) conditions.

If you have to fly in the heat this summer, use caution by paying attention to your aircraft’s performance. During this time of year, temperatures and humidity can reach highs such that the type of flight with a full load that you might make safely during the winter would not be safe to make during the summer.

Under these conditions, sometimes the best way to calculate performance is to take the worst value from the performance chart (regardless of any of the conditions for the flight) and increase it by 10-20 per cent. The added safety margin helps assure the safety of your intended operation and provide for a margin of error.

Please also calculate the actual expected performance so you know how well the aircraft can be expected to perform in a pinch.

Regardless of how you approach your performance calculations, be especially cautious about early morning departures with the intention of a mid-day return. Performance can decrease substantially by mid-day and make the return flight more difficult.

In the end, one safe way to view summertime temperature effects on performance is to first think about its effects on your own body. If you think you would have a hard time in the heat, it’s likely your aircraft would as well!This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 160 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.