Density altitude for turboprop pilots

Turboprops are forgiving airplanes right up until a summer afternoon at a 5,000 ft field, when the book number you remember from sea level turns out to be a thousand feet short. JetDesk puts a runway estimate on every airport page so the surprise happens on the screen, not on the roll.

01What density altitude is doing

Density altitude is the altitude the airplane thinks it is at. Heat and low pressure thin the air; thin air means less thrust from the propeller, less lift from the wing, and a higher true airspeed for the same indicated rotation speed, which means more runway. A 2,000 ft field on a 35°C day behaves like a 4,500 ft field on a standard day.

JetDesk computes density altitude from the live METAR (temperature and altimeter) and the field elevation, and shows it under the weather block. A DA more than 2,000 ft above the field turns the line amber.

02The rules of thumb JetDesk uses

• Takeoff distance grows about 8 percent per 1,000 ft of density altitude for turboprops. Piston airplanes are worse, jets somewhat better.
• Landing distance grows about 5 percent per 1,000 ft. Landing is less sensitive because the engine matters less on the way down.
• Headwind shortens both by roughly 10 percent per 9 knots. Tailwind lengthens them by roughly 10 percent per 2 knots, which is why a 10 knot tailwind is a different airplane.

These are FAA rule-of-thumb factors applied to the sea level, ISA, max weight, 50 ft obstacle numbers you enter in Settings for your airplane. They are estimates for planning. The POH performance charts for your serial number are the authority, and JetDesk says so next to every number.

03Why 50 percent

Book numbers come from a new airplane, a test pilot and a dry runway. Most operators plan with a factor of 1.5 on the landing distance (14 CFR 135.385 requires 1.67 on a dry runway for turbojets, and another 15 percent on top of that when the runway is wet). JetDesk shows the raw estimate and the 50 percent figure, and grades each runway: green at 1.5 times or better, amber down to 1.2, red under that. Red is not a prohibition; it is a reason to open the POH and think.

04The practical habits

Look at the DA line before the runway line. On a hot day, prefer the longer runway even with a small crosswind. If the estimate is amber, plan the departure for the morning. And enter your real book numbers once: presets load typical figures for a Meridian, M600, PC-12, TBM, King Air 350 and the light jets, but your airplane's POH is a few clicks away and worth the two minutes.

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