DUAL BLEED Light
The DUAL BLEED light illuminates whenever the APU bleed air valve is open and the
position of the engine bleed air switches and isolation valve would permit possible
backpressure of the APU. Thrust must be limited to idle with the DUAL BLEED light on.
Air Conditioning Packs
The flow of bleed air from the main bleed air duct through each air conditioning pack is
controlled by the respective pack valve. Normally the left pack uses bleed air from engine
No. 1 and the right pack uses bleed air from engine No. 2. A single pack is capable of
maintaining pressurization and acceptable temperatures throughout the airplane up to the
maximum certified ceiling. The APU is capable of supplying bleed air for two packs on the
ground, or one pack in flight. Most external air carts are capable of supplying adequate
bleed air for two pack operation. Do not operate more than one pack from one engine at
any time.
Airflow Control
With both air conditioning pack switches in AUTO and both packs operating, the packs
provide “normal air flow”. However, with one pack not operating, the other pack
automatically switches to “high air flow” in order to maintain the necessary ventilation rate.
This automatic switching is inhibited when the airplane is on the ground or inflight with the
flaps extended to insure adequate engine power for single engine operation. Automatic
switching to “high air flow” occurs if both engine bleed air switches are OFF and the APU
bleed air switch is ON regardless of flap position, air/ground status or number of packs
operating. With the air conditioning pack switch in HIGH the pack provides “high air flow”.
Additionally, an “APU high air flow” rate is available when the airplane is on the ground, the
APU bleed air switch is ON and either or both pack switches are positioned to HIGH. This
mode is designed to provide maximum airflow when the APU is the only source of bleed air.
