A dispatcher is computing maximum allowable takeoff weight for a flight departing a high-elevation airport on a hot afternoon. Although climb-limited and field-length-limited weights would permit a higher takeoff weight, the weight must be further reduced because the calculated ground speed at VR would exceed the aircraft's certificated tire speed limit. What is the primary reason high density altitude drives up ground speed for a given rotation speed?
- ATrue airspeed increases relative to indicated/calibrated airspeed as air density decreases, requiring a higher ground speed to achieve the same rotation speed
- BTire pressure decreases with altitude, reducing the tire's rated speed limit
- CWind direction changes predictably with elevation, increasing the headwind component
- DEngine thrust output increases with altitude, requiring higher rotation speeds
Show answer & explanationAnswer & explanation
Correct answer: A. True airspeed increases relative to indicated/calibrated airspeed as air density decreases, requiring a higher ground speed to achieve the same rotation speed
As density altitude increases, air density decreases, causing true airspeed (and therefore ground speed in no-wind conditions) to be higher than indicated/calibrated airspeed for any given rotation speed (VR). This higher ground speed at rotation can exceed the tire's certificated maximum speed, forcing a tire-speed-limited weight reduction (fewer passengers/less fuel changes VR but the density effect on TAS/groundspeed is the driver).
Why the other options are wrong
- B. Tire pressure is maintained per servicing schedules and isn't directly reduced by altitude in this context.
- C. Wind is not a predictable function of elevation and isn't the density altitude mechanism.
- D. Thrust output actually decreases with altitude and heat, it does not drive rotation speed up.
Tire Speed Limited Weight
A takeoff weight limitation imposed when high density altitude causes ground speed at rotation to approach or exceed the tire's certificated maximum speed.
- High density altitude increases TAS for a given IAS
- Higher TAS/ground speed at VR can exceed tire speed limits
- Reducing takeoff weight lowers required VR and ground speed
Memory trick: Thin air means fast tires — hot and high means slow down your weight.