In flight training we simulate many things. A departure stall, for example, is simulated at altitude but is meant to develop habit patterns that might be needed immediately after takeoff. As instructors, we orchestrate conditions and situations that allow our students to learn without affecting the safety of the flight. One example is creating conditions that resemble high-density-altitude operations even at sea level.
We must begin by making sure that students understand exactly what density altitude is and how it affects aircraft performance. Start with some ground school. Students may find the concept of density altitude confusing. Make sure they understand that density altitude is not an altitude that we can climb and maintain. Rather, it is a theoretical altitude that allows us to calculate how the aircraft will perform under certain circumstances. It is defined as pressure altitude corrected for nonstandard temperature. The higher the altitude and temperature, the less dense the air, and the greater the loss of aircraft performance.
Terminology aside, make sure students understand the concept of density altitude?how to define, calculate, and use it?before beginning any in-flight simulation. After some demonstrations and practice, your students should be able to work out density altitude problems using a flight computer or density altitude chart without your help. Make sure they also know how to calculate aircraft performance figures, including takeoff and landing distances and climb rates.
Next, explain how the thinner air at high density altitudes reduces aircraft performance. First, it reduces the aerodynamic lift over the wings and other airfoils. Second, it reduces the thrust from the propeller. (Students may forget that the propeller is also an airfoil. The propeller just generates its lift horizontally instead of vertically.) And third, with less oxygen available for combustion, a normally aspirated engine produces less power as density altitude increases. (A normally aspirated engine loses roughly 2 percent of its horsepower for each 1,000-foot increase in altitude.)
These three factors combine to cause the loss-of-performance effects pilots see and feel from the cockpit. At high density altitudes, students will notice slower acceleration on the takeoff run, a decreased rate of climb, and, for the same indicated airspeed, a faster ground speed on takeoff and landing.
To effectively simulate high density altitude, we must find ways to safely reproduce these performance symptoms. First, choose an appropriate airport for practice. Since takeoff ground runs and landing rollouts may be significantly longer than usual, select an airport with a long runway, preferably 6,000 feet or more. The approach and departure zones should be free of obstacles to maximize ground clearance at the reduced rate of climb.
To simulate the loss of engine power that results from high density altitudes, perform a reduced power takeoff. Consult the climb performance charts in the aircraft?s pilot?s operating handbook (POH) and find the climb rate at the altitude you wish to simulate. Then set the engine power to produce this climb rate. For example, say a typical trainer climbs at 700 feet per minute at sea level under standard conditions. At 6,000 feet, this climb rate decreases to 425 feet per minute. Therefore, to simulate a departure from an airport with an elevation of 6,000 feet, power could be set to produce a 425-foot-per-minute climb rate.
Of course, one of the most realistic ways to simulate the effects of high density altitude in a typical four-seat trainer is to load it up to the maximum takeoff weight. This means filling the fuel tanks, putting a couple of passengers in the rear seats, and then (after a careful calculation of weight and balance), giving your student some dual instruction in takeoffs, landings, go-arounds, and emergency procedures. The added weight will slow acceleration, reduce the climb rate, and increase takeoff and landing distances.
The effect of high density altitude on ground speed can be significant, especially when the temperature is above standard. For example, a typical summertime temperature at Big Bear Airport is 20 degrees Celsius. This equates to a density altitude of approximately 9,000 feet. During a 60-knot indicated airspeed approach, the true airspeed (and ground speed in zero wind) is roughly 69 knots, a 15-percent increase. To simulate this effect, a normal (60 knot) approach and landing could be made at sea level with a slight tailwind (less than 10 knots). The perspective from the cockpit would be a fair representation of what it would be like to land at a density altitude of 9,000 feet. Most POHs include takeoff and landing distance data for operations with tailwinds of up to 10 knots to help you determine necessary runway length.
When teaching your students how to deal with the effects of high density altitude, it is helpful to emphasize some key concepts. Make sure to teach your students how to lean the mixture to maximize engine power output. Because the aircraft will be operating with less power, teach students to become drag conscious. For example, failure to retract the flaps according to the manufacturer?s specifications on a go-around could result in no climb performance at all.
Use the checklists. Airspeeds should be flown with tight tolerances. Control pressures should be applied smoothly. And pilots must exercise patience. The aircraft will respond, but it may take a little more time to react. Finally, make sure to discuss the kinds of decisions that might have to be made when operating at high-density-altitude airports.
Consideration should be given to performance when planning the flight and making weight-and-balance calculations. For example, because landing distances do not increase in the same proportion as takeoff distances, it is possible to land at a high-density-altitude airport and then not have enough runway for takeoff. That would be a classic one-way trip. In that case, it might mean waiting until early morning to depart when ambient temperatures are cooler and density altitude is lower.
Consideration should also be given to approximate takeoff abort points, in case the aircraft is not accelerating properly. Take into account critical items such as obstacles and emergency returns. Because go-around climb performance is reduced, the decision to go around needs to be made early.
Of course, the effects of high density altitude are most noticeable during takeoff and climb. Slower acceleration means it takes more time to reach rotation speed, the speed over the ground is higher than usual, and the aircraft uses a lot of runway. A common problem during a high-density-altitude takeoff is rotating early. This increases induced drag (a byproduct of lift) and delays acceleration to liftoff speed. Students may begin the rotation based on ground speed rather than the airspeed indicator, or they may rotate early because the aircraft is covering a lot of runway on the takeoff roll. At high density altitudes, increased takeoff distances are the norm. Students should do preflight planning to gain a good idea of how much runway the aircraft will use on the takeoff roll.
Another common problem in high-density-altitude takeoffs is trying to pull the aircraft off the runway at rotation because students don?t experience the normal brisk liftoff. Remind them to be patient, rotate smoothly to the normal pitch attitude, and then let the aircraft fly itself off the runway when it is ready. Once airborne, have them hold the proper pitch attitude while allowing the aircraft to accelerate to the best angle or best rate of climb airspeed. Because of the reduced climb performance, students may continue to increase the pitch attitude until the stall warning horn activates. Attempting to climb out of ground effect before achieving the best rate-of-climb airspeed can result in the aircraft settling.
Students also may forget to check the ball in the inclinometer to maintain proper rudder coordination. Because there is no longer a lot of excess power available for climb, reducing drag becomes very important. So, to reduce form drag (the drag caused by the frontal area of the aircraft), the aircraft must be flown coordinated and without any sideslip. Uncoordinated flight, as indicated by a ball out of center, can easily steal 200 to 300 feet per minute of climb.
Close to the ground at high density altitude, students can misinterpret the faster approach speed as a greater descent rate. As a result, they may flare too early. To compensate for this, remind students to use their peripheral vision and be vigilant about flying at the proper approach indicated airspeed. To maintain a safe, stabilized glide path, take advantage of any glide path indicators available, such as a visual approach slope indicator (VASI) or precision approach path indicator (PAPI). Additionally, remember that landing rollout distances will be greater because of the increased ground speed at touchdown.
If you and your student attempt a go-around, be prepared for lackluster climb performance. To avoid surprises, consult the POH during preflight planning to determine climb performance at altitude. Further, students should be familiar with the concept of delaying the extension of flaps (which causes excess drag) until the landing is assured. It also means making an early decision to go around if the approach doesn?t look right.
Students can obtain their private pilot certificates without ever knowing how an aircraft behaves and performs in high-density-altitude conditions. But armed with some clever methods of contriving experiences, even instructors at low-elevation airports can give their students an idea of what high-density-altitude operations are all about. It is invaluable training that will definitely impart a great respect for aircraft performance.