Notes: This first of a two-article series will focus on density altitude and aircraft performance. The second will focus on other factors such as rate of climb, angle of climb, and takeoff performance.
I like spontaneity, particularly when it’s well-planned.
Safe flying often involves knowing what is coming our way, so we don’t have to make critical last-minute decisions in tight situations. Dodging an unexpected power line. Recovering from a near stall/spin due to violent wind shear. Curling the toes as we make it over a line of trees because a slight tailwind made for an all too exciting moment. Trying to decide if we are too close to susceptible crops to ensure no drift.
As the old saying goes, a superior pilot uses superior judgment to stay out of situations requiring superior skill. Whether it’s aircraft performance or weather or crop stage, it’s good to know factors affecting aircraft performance. Many we can anticipate. After an enjoyable morning at home base flying with cool, dense air providing thrust and lift in spades, the afternoon brings rising temperatures that make for a noticeable decrease in aircraft performance. No surprises there, but if the load size is not adjusted to accommodate a significant temperature change, things can get pretty exciting really quickly.
Or you’ve traveled several hundred miles away from the home strip to a large-scale forestry spraying operation where the legs can be many miles in length and cover territory where there are considerable altitude differences in the various blocks to be sprayed. A hot rod of an aircraft in one block can turn into a dog in a much higher spray block as airspeed dissipates climbing to the higher terrain, but again, it’s something we can easily plan for.
One of the nice things about ag flying is that you can easily adjust the load size to fit the conditions. I had the opportunity to work with an experienced ag operator out of Arizona. His rule of thumb was that at 85 F, he automatically reduced the maximum load size to allow for decreased performance e.g., an AT-401 would change load size from the standard load of 360 g to 320 g.
It’s not just for takeoff and climb, but more to the point, if you are working smaller fields with lots of back-to-back turning, carrying too heavy a load for the conditions makes it feel like you are balancing on top of a beach ball, with mushy controls and a struggle to keep a safe airspeed until the load lightens. It is really uncomfortable flying ‘on the edge’ near stall/spin conditions, as well as being very fatiguing.
It’s easy to remember to match load size to conditions when things are going along well, but when you have a long line of farmers pleading with you to treat their crops now invested with a large-scale insect infestation, there is a tendency to push parameters because of the added urgency.
Many factors have an effect on aircraft performance, and it is the total of these effects that are good to know ahead of time. In fact, the total of these effects can be very significant in terms of decreased aircraft performance in terms of engine power available, propellor thrust available, and the amount of lift available from the wings. Knowing how these dictate safe load size and/or climb performance ensures the only surprises you get are pleasant ones.
Density Altitude – A Quick Review
As the density of the air increases (lower density altitude), aircraft performance increases, and as air density decreases (higher density altitude), aircraft performance decreases. No surprises here, but what is surprising is how much even a small temperature change can affect density altitude.
Using Figure 11-22 (courtesy FAA Pilot’s Handbook of Aeronautical Knowledge), start with an airport elevation of 5,883 ft MSL, a temperature of 70 F and an altimeter setting of 30.10 “Hg.
Locate 30.10 under the altimeter heading and read across to the second column, which reads “–165.” Subtract 165 from the airport elevation to give a pressure altitude of 5,718 feet. On the scale along the bottom of the graph, locate 70° and draw a line up to the 5,718 feet pressure altitude line, which is about two-thirds of the way up between the 5,000 and 6,000-foot lines. Draw a line straight across to the far left side of the graph and read the approximate density altitude of 7700 ft.
How High Are You Flying
The significance? The aircraft will perform as though it were at 7700 feet, with quite a performance decrease from the actual altitude of 5718 ft. Now put that into terms an ag pilot can understand. The difference is almost exactly 2000 feet in altitude. That means that given the conditions in the above example, when you’re spraying at ground level, the aircraft has the decreased performance equivalent to an aircraft flying at 2000 feet. It’s really something you need to consider, so no nasty surprises come your way.
Ignoring the perils of a high-density altitude can get both experienced and inexperienced pilots into trouble, particularly true in hot weather and at higher elevations, but even when at your home strip, when the action gets hot and heavy due to a sudden bug infestation, safety may be compromised in the pressure of the moment to get the job done.
To recap, high-density altitude reduces all aircraft performance parameters. This means that the normal horsepower output is reduced, propeller efficiency is reduced and a higher true airspeed is
required to sustain the aircraft throughout its operating parameters. It means an increase in runway
length for takeoff and landings, a decreased rate of climb, and a distinct challenge in keeping the airspeed from dropping below a comfortable level during aerial applications. It also means giving obstacles a larger-than-normal margin of safety
Interestingly enough, at airports with elevations of 2,000 feet and higher, control towers and FSSs will broadcast the advisory “Check Density Altitude” when the temperature reaches a predetermined level as a reminder to pilots that high temperatures and high field elevations will cause significant changes in aircraft characteristics.
So, wherever you’re operating, whatever the job at hand, just keep in mind the whole concept of density altitude. You may be flying higher than you think.