Final answer:
After engine start with the APU BLEED pb ON, the Auxiliary Power Unit (APU) is still supplying bleed air to the air conditioning packs until the engines are stable enough to take over. The APU provides pneumatic power for cabin pressurization and air conditioning while the aircraft is on the ground or the main engines are not running.
Step-by-step explanation:
The APU (Auxiliary Power Unit) is a device on an aircraft that provides energy for functions other than propulsion. When the question asks 'After engine start with the APU BLEED pb ON, what source is supplying bleed air to the packs?', it is related to the pneumatic system of the aircraft. After engine start, if the APU BLEED push button (pb) is ON, the APU is still providing bleed air to the air conditioning packs until the engines are ready to take over the supply.
Usually, after the engines have started and stabilized, pilots switch the source of bleed air from the APU to the engines' bleed systems. This transition ensures the APU is not unnecessarily used when the engines themselves can supply the required bleed air to pressurize and condition the cabin. The specific procedures and the point at which the switch is made from APU bleed to engine bleed can vary based on the aircraft type and operational procedures.