Question

How does an increase in density altitude (lower air density) affect the take-off ground roll distance (\(S_g\))?

• A. It decreases the distance due to reduced aerodynamic drag
• B. It has no effect on the distance as weight remains constant
• C. It increases the distance because the required True Airspeed for lift-off increases and also the engine thrust decreases
• D. It decreases the distance because the coefficient of friction is lower in thinner air

Hint:

Higher density altitude implies \[ \boxed{ \begin{aligned} \text{Lower air density} &\Rightarrow \text{Less lift} &\Rightarrow \text{Less thrust} &\Rightarrow \text{Longer take-off distance} \end{aligned} } \] A useful pilot's rule is: \[ \boxed{\text{High, Hot and Humid } \Rightarrow \text{Poor aircraft performance}.} \]

Answer 1

The Correct Option is C

Step 1: Recall the effect of density altitude. An increase in density altitude means \[ \boxed{\text{air density decreases}.} \] Lower air density causes
• reduced engine thrust,
• reduced propeller efficiency,
• reduced wing lift at a given true airspeed.

Step 2: Determine the effect on take-off distance. Since the aircraft produces less lift at the same speed, \[ \boxed{\text{a higher True Airspeed is required for lift-off}.} \] Also, because the engine develops less thrust, \[ \boxed{\text{the aircraft accelerates more slowly}.} \] Therefore, the take-off ground roll distance increases. Hence, \[ \boxed{\text{It increases the distance because the required True Airspeed for lift-off increases and also the engine thrust decreases}.} \] Thus, \[ \boxed{(C)} \] is the correct answer.