Auxiliary power unit operation during takeoff is a widely practiced but often airline-specific procedure that reflects the industry's layered approach to redundancy during the most critical phase of flight. While ground crews typically start the APU to provide electrical power and bleed air before engine start, many operators explicitly direct crews to leave the APU running through the takeoff roll and initial climb, with an "APU off" item appearing on the after-takeoff or climb checklist only once the aircraft is established and all primary systems are confirmed stable. This is not an anomaly or oversight — it is a deliberate design philosophy built into standard operating procedures at numerous major carriers.
The primary rationale for APU-on takeoff operations is electrical redundancy. Commercial transport aircraft rely on engine-driven integrated drive generators (IDGs) for primary electrical power once airborne, but during the takeoff roll and rotation, crews have very limited time to respond to a dual-generator failure. With the APU generator active and connected to the electrical bus, a failure of one or both IDGs does not immediately cascade into a loss of essential avionics, flight control computers, or display systems. On fly-by-wire platforms such as the Airbus A320 family, where electrical power is foundational to flight control law authority, this redundancy is particularly meaningful. On Boeing narrowbodies operating under aggressive airline reliability standards, the same logic applies.
Bleed air is a secondary but relevant factor. On most modern aircraft, APU bleed is deselected or automatically inhibited during takeoff to avoid robbing engine performance — particularly important at high-elevation airports like Denver International (KDEN) or Quito (SEQM) where density altitude already degrades thrust margins. However, at lower-elevation airports or under specific MEL provisions, some airlines permit or require APU bleed to remain available as a contingency source for pressurization and air conditioning should an engine bleed failure occur during takeoff. Aircraft-specific differences matter considerably here; the Boeing 787 Dreamliner, which uses an all-electric bleed-free architecture, has a different APU role than a conventional pneumatic-architecture aircraft like the A321.
For Part 135 and Part 91K operators flying business jets, APU practices mirror commercial logic but scale differently. On aircraft such as the Gulfstream G650, Bombardier Global 7500, or Dassault Falcon 8X, APU-on takeoff is sometimes mandated by aircraft flight manual limitations or company SOPs when operating in certain configurations — for example, when an IDG is deferred under MEL or when operating from high-altitude airports where engine restart capability after a rejected takeoff is a concern. Crews flying these platforms should understand that APU procedures are not one-size-fits-all; they are embedded in aircraft-specific performance and systems logic and reflect real-world failure mode analysis.
The broader takeaway for professional pilots reviewing this question is that checklists tell a story about the failure modes an operator most wants to guard against. An "APU off" item appearing after takeoff rather than before it reveals an intentional choice to keep a backup power source alive through the highest-workload, lowest-altitude window of flight, then shed it once the situation is more manageable. Understanding the reasoning behind each checklist action — not just the action itself — is a core element of systems mastery and situational awareness, and this particular item is a textbook example of redundancy architecture translated directly into daily airline operations.