True airspeed increases about 2% per 1000 ft of density altitude. If TAS at sea level is 120 knots, what is TAS at 10,000' density altitude?

True airspeed increases about 2% per 1000 ft of density altitude. If TAS at sea level is 120 knots, what is TAS at 10,000' density altitude?

• A. 120 knots
• B. 132 knots
• C. 144 knots
• D. 156 knots

Answer: (C)

Density altitude tells us how thin the air is; as it increases, the air offers less mass for the wings to push against, so true airspeed rises for a given indicated airspeed. The rule of thumb here is a 2% increase in TAS for every 1,000 ft of density altitude. Ten thousand feet is ten increments of 1,000 ft, so TAS increases by about 20%. With a sea-level TAS of 120 knots, multiply by 1.20: 120 × 1.20 = 144 knots. So the true airspeed at 10,000 ft density altitude is about 144 knots.

Density altitude tells us how thin the air is; as it increases, the air offers less mass for the wings to push against, so true airspeed rises for a given indicated airspeed. The rule of thumb here is a 2% increase in TAS for every 1,000 ft of density altitude.

Ten thousand feet is ten increments of 1,000 ft, so TAS increases by about 20%. With a sea-level TAS of 120 knots, multiply by 1.20:

120 × 1.20 = 144 knots.

So the true airspeed at 10,000 ft density altitude is about 144 knots.