Density altitude
On a hot afternoon your 3,000-foot field performs like somewhere much higher. The tug climbs worse, the ground roll is longer, and your approach is faster over the ground than the airspeed indicator suggests. This works out by how much.
Density altitude
Pressure altitude
ISA deviation
Density ratio σ
TAS / IAS
What this means on the day
- The tug will climb worse and the ground roll will be longer. Both the engine's power output and the wing's lift fall with air density, so the whole launch is degraded at both ends of the rope. Apply your towplane's flight-manual factors — we are not going to guess a climb rate for an aircraft we know nothing about.
- Your true airspeed is higher than indicated. You fly the same numbers on the ASI, but you are moving faster over the ground. The approach feels quicker, the ground rush is greater and the float in the flare is longer.
- Take-off and landing distances both grow. If your field is short, this is the day it matters.
- This is a high density altitude day. The FAA's Glider Flying Handbook discusses launch performance at altitude, and clubs operating regularly above roughly 4,000 ft density altitude generally want a more powerful towplane than the same operation at sea level. If your tug is marginal, today is not the day to find out.
What this assumes, and what it can't know
This is standard-atmosphere arithmetic, and it is exact. Pressure altitude from your altimeter setting, ISA deviation from the temperature, and the density ratio from the standard atmosphere model. There is nothing estimated in the four numbers above.
What it deliberately does not tell you is your climb rate or your ground roll. Those are specific to the aircraft, its weight, its propeller and the pilot's technique. Printing a number for them would turn an advisory into a performance assertion about somebody else's towplane, and it is exactly the kind of claim that gets believed on the wrong day. Your flight manual has the factors.
Humidity is ignored. Moist air is slightly less dense than dry air, so a very humid day is marginally worse than this shows. The effect is small next to temperature and pressure and it is conventionally left out of density altitude.
Use the temperature at the field, now. Not the forecast high, not the reading from the shaded clubhouse porch. On a hot afternoon a runway thermometer and a shaded one can differ by several degrees, and several degrees is several hundred feet.
Not yet reviewed by anyone but us. If you run a club's books and one of these assumptions is wrong, we would genuinely rather hear it than not — tell us and we'll credit you here. This is a budgeting estimate, not a quote. Every club sets its own rates and they change; check with the club before relying on a number.