Leaning and Mixture on a High Density Altitude Departure
You are departing a strip at 4,200 ft elevation on a hot day, with the temperature 30 degrees C, giving a density altitude near 6,500 ft. The strip is 700 m of sealed surface with rising terrain 1 nm beyond the departure end. Your aircraft is a normally aspirated light single with a fixed-pitch propeller, and you are carrying two adults and full fuel. The pilot operating handbook gives a take-off ground roll of 320 m at sea level, standard temperature, maximum weight, and a climb rate of 700 ft per minute in the same conditions. You have flown this strip before in cooler weather.
1. How does density altitude change the take-off and climb, and what should you do about the mixture?
Read the worked answer
High density altitude reduces air density, so the engine produces less power, the propeller produces less thrust, and the wings produce less lift for a given true airspeed. The result is a longer ground roll, a lower rate of climb, and a higher true airspeed for the same indicated climb speed. The handbook figures at sea level standard do not apply; you must use the density altitude charts in the handbook, which will show a substantially longer roll and a much reduced climb. Lean the mixture for best power before take-off as the handbook directs, because at 6,500 ft density altitude a full-rich setting will be over-rich and further reduce power. Do not simply apply the sea-level figures and hope.
2. Given rising terrain 1 nm beyond the departure end, what is your decision?
Read the worked answer
The decision should be to reduce weight and reconsider the departure, or to use a different departure path. Two adults and full fuel at near maximum weight on a hot high strip with rising terrain leaves little margin if the climb is below the handbook figure. Options are to reduce fuel to the minimum needed for the flight plus a sensible reserve, offload baggage or a passenger, wait for a cooler part of the day, or depart on a route that avoids the rising terrain. If none of these is possible, the correct decision is not to depart. The key point is that the climb gradient, not just the ground roll, must clear the terrain, and a reduced climb rate on a hot day can turn a normal departure into an accident.
3. What pre-flight calculations and checks would you complete before deciding?
Read the worked answer
Calculate the density altitude from elevation and temperature, then use the handbook charts to find the take-off distance and climb rate at the actual weight, and compare them with the available strip length and the terrain clearance required. Include the effect of wind and surface, and add a margin for the pilot and aircraft. Check the aircraft weight and balance, confirm the mixture setting for the altitude, and review the engine and propeller limitations. Check the emergency plan if the engine fails after lift-off, given the rising terrain. Record the figures so the decision is evidence-based rather than a feeling, and be prepared to cancel if the numbers do not give a comfortable margin.