Notes on the use of APU/Engine Air for Conditioning Cabin

[A Former User]

As per the thread title. I hope these notes are of interest to the forum.

The use of APU air or engine during operations is dictated by:

1. The aircraft manufacturer.
2. The Minimum Equipment List (MEL).
3. Company procedures.

After Start

It is recommended that both packs are used after engine start so that pack temperature control will stabilise before take-off.

Before switching the packs on, the APU AIR or ENG AIR must be ON to open the discharge or outflow valves.

In aircraft with fully automatic pressurization systems: [as modelled on the JF146 simulator]

• One PACK may be selected ON once one outflow valve indicates open.
• Both PACKs may be selected ON when both outflow valves indicate open.

After the first pack is selected ON, check that the APU TGT/EGT has stabilised before selecting the second pack ON.

If the take-off is to be made with conditioning the aircraft from the engines, the CABIN AIR must be selected to RECIRC.

If APU AIR is not available and engine air is not to be used for air conditioning, select just ENG 4 AIR ON inorder to ensure that the discharge valves open.

Use of APU Air in Flight

The APU air can supply both packs up to 15,000 ft. During normal operation, it would not usually be used above 5,000 ft.

As altitude is increased the mass flow delivered by the APU may not be sufficient to keep both PACK VALVE annunciators extinguished, although the flow rate will be adequate for pressurisation. If this condition is met, revert to engine air as soon as practicable to improve cabin ventilation.

If the APU AIR is used for take-off, change over to engine air when the engines have stabilised at climb power. Avoid introducing engine air at high engine power.

The APU may then be shut-down, However, since the APU is required under some failure and MEL conditions, the APU must not be stopped until it is confirmed by both Pilots that the APU is no longer required. If the sector flight time is less than 20 minutes, make the air changeover to the engines as normal after take-off, but the APU may be kept running to reduce crew workload. Another start cycle is worth about 20 minutes of running from an APU life point of view.

If airframe ice protection is required, the air changeover must be made as soon as the airframe ice protection is required. This may require a reduction to climb power earlier than normal.

Always confirm that the aircraft is pressurising correctly early in the climb (and passing at 10,000 and 20,000 ft) by monitoring cabin altitude, vertical speed and differential pressure. There is no direct warning that the PACK's are both OFF and that the pressurization system has failed to pressurize until the CAB HI ALT warning activates.

If the APU AIR is to be used for the approach or after landing, start the APU at least five minutes before the air changeover is required.

If the APU is used for conditioning during the approach and landing, then all the ENG AIR switches are selected OFF for landing. APU AIR must not be used until the WING and TAIL ice protection switches are OFF.

Use of Engine Air on the Ground

The engines may be used for conditioning after engine start and after landing. There are two operational points:

• The PACK VALVE/ENG VALVE annunciators may illuminate at low N2 due to low flow. Increasing N2 will eventually achieve flow that will extinguish the annunciators. However, nuisance MWS cautions may be given on thrust reduction. On some aircraft, a small increase in N2 will cause the annunciators to extinguish and to remain extinguished at ground idle. However, caution must be exercised so that excessive jet blast is avoided.

• In tailwinds, jet exhaust may be re-ingested; it may be necessary to select the packs off to prevent unpleasant smells in the cabin.

Use of Engine Air for Take-off and Landing

A few individual 146 aircraft received approval to use engine air to supply the air conditioning on take-off. These aircraft can be identified by a statement on the flight deck placard. This would not apply to any of the JF146 models.

Take-off and Landing without Air Conditioning

The no-conditioning configuration requires both packs and all ENG AIR to be off except ENG AIR 4; air from engine 4 is required to control the discharge valves or outflow valves - there is no performance penalty. On take-off or go-around, conditioning is introduced after thrust reduction. It is important to follow the switching sequence in the procedure so as to minimises pressurization disturbances. Note. If for any reason engine 3 is not available, use another engine for the supply.

The switch settings for take-off or go-around without air conditioning are:

• Both PACK switches OFF.
• ENG AIR 1, 2 and 3 switches to OFF.
• ENG AIR 4 switch to ON.

After thrust reduction altitude: [Thrust reduction is normally a minimum of 400ft AGL]

• Select PACK 2 switch to ON.
• Let the pressurization stabilise, then select ENG AIR 1, 2 and 3 switches to ON.
• Let the pressurization stabilise, then select PACK 1 switch to ON.

On an approach for a landing without air conditioning, the packs and engine air are normally switched off by 500 ft above the threshold, and always before starting a go-around.

[A Former User]

Apologies for an error in the above - having a senior moment between finger and brain… too late to correct as more that 3 hours elapsed since original posting.

The thrust reduction during a normal take-off follows completion of flap retraction. Acceleration from V2 +10 to progress through the flap retraction schedule usually commences from a minimum of 400 ft agl. However, thrust reduction ‘cut-off’ may commence as early as 800 ft agl during a noise abatement climb.