How does temperature deviation from ISA influence airspeed readings?

• A. IAS becomes more accurate.
• B. IAS is unaffected.
• C. IAS becomes slower.
• D. IAS may be inaccurate because the relationship between pressure and speed depends on air density; ISA deviations cause errors.

Answer: (D)

Temperature deviations from ISA change air density, and airspeed indications are tied to dynamic pressure, which depends on density. The Pitot tube measures dynamic pressure (q = 1/2 ρ V^2), and the IAS scale converts that pressure to a speed using standard (ISA) density. If the air is warmer than ISA, density is lower, so for a given true airspeed the dynamic pressure is lower and the IAS reads lower than actual. If the air is colder, density is higher, increasing dynamic pressure and making the IAS read higher. Since the conversion assumes ISA density, deviations in temperature cause IAS to be inaccurate.