Airbus A220 APU and Fire Protection

The Airbus A220 has one auxiliary power unit (APU), a Honeywell 131-9(C) on both models, which supplies electrical power and bleed air. Its fire protection watches the engines and the APU for fire, the cargo compartments, lavatories and equipment bay for smoke and the main landing gear bays for overheat. Extinguishers serve the engines, the APU, the cargo compartments and the lavatories, and from the flight deck the crew can extinguish a fire in either engine, the APU or the cargo compartment.

The public documents used here, the EASA type-certificate data sheet (TCDS), the Transport Canada (TCCA) MMEL and evaluation report, the FAA Flight Standardization Board report and the Airbus Aircraft Characteristics, show the controls, the components and the dispatch rules, but not the fire drills of the flight crew operating manual. Dispatch conditions quoted are the MMEL's; an operator's MEL may be more restrictive. The APU's electrical and bleed roles are covered in A220 electrical system and A220 air conditioning, pressurisation and oxygen, and the principles in auxiliary power unit (APU), fire and overheat detection, engine fire protection and cargo and lavatory fire protection.

The APU and what it supplies

Both model sections of the EASA TCDS list one Honeywell 131-9(C) APU and refer to the AFM for its operating limits. The APU is a source of both electrical power and bleed air. When its APU bleed air valve is inoperative, secured closed with APU BLEED selected OFF, the MMEL notes that the APU is still available as a source of electrical power, and the relief for the APU BLEED FAIL caution says the same.

The MMEL leans on the APU generator when an engine generator is lost. Except for ETOPS, dispatch with one variable frequency generator (VFG) system selected OFF requires the APU to be started before departure and operated continuously throughout the flight, with all electrical power centres, all transformer rectifier units and the opposite VFG verified working (see A220 electrical system). How the APU bleed serves the packs and engine starts is described in A220 air conditioning, pressurisation and oxygen.

The Boeing 787 shows the alternative design: its APU gives no bleed air at all (see 787 no-bleed architecture and Boeing 787 APU and fire protection).

APU controls, start and limits

The APU switch is on the APU and TAWS control panel. Besides the flight deck controls, the MMEL lists two APU shutdown switches, one on the external service panel and one in the APU compartment.

The APU can normally be started on battery power: the MMEL warns that battery APU start may be inoperative with the message 24 ELECTRICAL FAULT - BPCU 2 DEGRADED, and another relief requires the APU start to be verified once each flight day. The APU exhaust gas temperature (EGT) is read on the SUMMARY synoptic page; avionics Build 8.0A2 corrected the readout logic so that the yellow indication no longer stays latched after an EGT exceedance.

Build 8.0A2 also added the APU OIL LO TEMP caution. When it posts, the AFM procedure has the crew start the APU. For operators with the ETOPS option with APU on demand, this removed the default need to start the APU during ETOPS missions.

The sources used here give no starting or operating altitude limits for the APU; the TCDS leaves its operating limits to the AFM. One ground recommendation does appear: Airbus does not recommend de-icing with the APU running, because ingested fluid can contaminate the air conditioning system, bring fumes and odours into the aircraft and possibly damage the APU. If the APU is running during de-icing, the APU shutoff valve is closed.

Engine fire detection

The detection systems are redundant. The MMEL's 26 FIRE SYSTEM FAULT INFO messages report a loss of redundancy in each of them: engine fire, APU fire and cargo smoke detection, main landing gear bay overheat detection and equipment bay smoke detection. The sources used here do not describe the engine detectors themselves.

Both model sections of the EASA TCDS record an equivalent safety finding, E-13, titled "Fan Zone Non Fire Zone": the engine fan zone is treated as a non-fire zone. EASA's findings for the A320neo likewise leave the fan compartment of its PW1100G-JM, also a geared turbofan, out of the fire zones: the one designated fire zone is the core (see A320 fire and smoke protection).

The fire controls are on the overhead ENGINE & APU FIRE panel: guarded pushbutton annunciators (PBAs) for the left engine, the APU and the right engine, labelled L ENG, APU and R ENG. The A220 therefore has no fire handles in the Boeing 737 sense; like the A320, it uses fire pushbuttons. The sources used here do not list what pressing an engine fire PBA closes or arms. In general, a fire handle or pushbutton isolates the engine's fuel, hydraulic, bleed and electrical supplies, and a separate action fires a bottle.

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Engine fire extinguishing

The overhead panel carries five bottle PBAs for the engines and the APU: L ENG BTL 1, L ENG BTL 2, R ENG BTL 1, R ENG BTL 2 and APU BTL. Each engine thus has two bottle pushbuttons. Each PBA has two light functions in the MMEL: an AVAIL light and an amber light bar.

The bottles are fired by squibs (firing cartridges), and the MMEL shows that each extinguisher it names, L ENG, R ENG, APU, FWD CARGO and AFT CARGO BTL, has redundant squibs. A loss of squib redundancy gives a 26 FIRE SYSTEM FAULT INFO message, which may be displayed for dispatch. The sources do not say how many bottles are installed or where they are.

APU fire protection

The MMEL lists two APU fire detection loops. A loss of redundancy gives the INFO message 26 FIRE SYSTEM FAULT - APU FIRE DET REDUND LOSS; with the APU FIRE DET FAIL caution, the APU must be considered inoperative and not used. The APU's guarded APU FIRE PBA sits between the two engine PBAs on the ENGINE & APU FIRE panel, and the APU BTL PBA serves its extinguisher.

The APU bottle has its own dispatch rules. With APU BTL LO, an advisory, or APU BTL FAIL, a caution, the MMEL allows dispatch only with the APU considered inoperative and not used.

Cargo, lavatory, equipment bay and gear bay

Both underfloor cargo compartments, forward and aft, are sealed to meet the requirements of a Class C compartment, and their linings include provisions for compartment lighting, a smoke detector and fire extinguishing. A Class C hold, in the classification of CS 25.857, cannot be reached by the crew in flight, which is why it needs a detection system warning the flight deck and a built-in extinguishing system controlled from there (see cargo and lavatory fire protection).

The overhead CARGO BTL panel carries the guarded FWD FIRE and AFT FIRE PBAs and a CARGO BTL PBA, which has the same AVAIL light and amber light bar as the engine bottles. The dispatch rules depend on the message. With CARGO BTL FAIL, a caution, both compartments must be empty or free of combustible materials; with FWD CARGO BTL FAIL or AFT CARGO BTL FAIL, only the compartment concerned is restricted.

The MMEL ties the lavatories to the FIDEX control unit, FIDEX being Fire Detection and Extinguishing in the FAA's abbreviations. The unit has two channels: with channel A inoperative the forward lavatory is closed to passengers, with channel B the aft lavatory or lavatories, and in each case the other channel must be verified working. Each lavatory has a smoke detection system and a fire extinguishing system; a lavatory whose extinguishing system is inoperative stays in use only if its smoke detection works.

The equipment bay has two avionics bay smoke detectors. Several bleed air reliefs in the MMEL, for the fan air valve, an engine bleed PRSOV or a high pressure valve, require both to work.

The two main landing gear bay overheat detection loops have a striking relief. Except for ETOPS, dispatch with them inoperative requires the brakes to be inspected and found cool before each flight, and the landing gear to be left extended for at least ten minutes after take-off, with performance and fuel planned accordingly. After an engine failure past V1, performance comes first and the gear is retracted normally. The gear and brakes themselves are covered in A220 landing gear, brakes and steering.

Rescue and fire fighting

The Airbus Aircraft Characteristics include rescue data for airport fire services. They note that, in the flight deck, the control panels extinguish fire in the left engine, the right engine, the APU and the cargo compartment, and they recommend these agents to the fire services:

Messages and dispatch

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.