THE VIRGIN BUS

INTRODUCTION

Most jet transport aircraft are very poorly designed with regard to the protection, detection and effective handling of in-flight electrical fires, as the Swissair Flight 111 crash off Nova Scotia on 2nd September 1998 tragically demonstrated. 1

This situation is exacerbated by the widespread use in aircraft of a potentially dangerous electrical wire product called 'KAPTON' 2 and the fact that the current generation of airline jet transport aircraft (i.e. Airbus A320, A330, A340 and Boeing B777 and MD11) cannot be flown without electricity, unlike earlier generation jets such as the DC9.

The Virgin Bus

Inflight electrical fire would be best handled by the provision of a separate standby electrical wiring system which was powered by a totally separate Battery and Air Driven Generator (ADG). It would be used in the event of an in-flight electrical fire to power essential aircraft systems normally available under Standby Electrical power. (i.e. items normally located on the Standby Electrical Bus, Hot Battery Bus and Battery Bus)

In normal operation, the Virgin Electrical wiring system would remain disconnected and unpowered except for preflight checks.
In the event of smoke, fumes, cascading systems failure (i.e. a suspected electrical fire), the checklist procedure would be to pull an 'ELEC FIRE' handle which would mechanically accomplish a positioning of the Virgin Bus-tie relay time-delay and thereby:

  1. Physically disconnect the aircraft battery from circuit.
  2. Physically connect the 'virgin' battery and ADG power feeders to the virgin bus load centre to assume the powering of essential electrical items normally powered by the Standby Bus then
  3. Remove ALL power from the normal electrical wiring system. (by electrically tripping ALL generator field relays) and thereby resolve the smoke/fire emergency.

See detail at www.iasa.com.au/virginbuss.htm

Clearly the Virgin Bus proposal needs much design work to take into account numerous engineering considerations such as:

  • The routing of 'virgin' electrical bus wires as far away from all other electrical wiring as possible.
  • Reverse current protection to ensure that power from the 'virgin' bus is isolated and cannot enter into the normal electrical circuit via any emergency standby power items. (i.e. when they are being powered from the Virgin electrical bus)
  • The installation of a dedicated Virgin Bus circuit breaker panel easily accessible to aircrew in a dim, smoke-filled cockpit.
  • The installation throughout all sections of the aircraft structure of reliable electrical smoke and fire detector systems. As the pilot will need a fairly definite indication of a malfunction before making his decision, CCTV cameras in the E&E Bay and other strategic areas may be required.

Other significant design initiatives to reduce the risk of inflight electrical fire include:

  • The installation of fire-extinguishing systems throughout all vulnerable sections of the aircraft structure, especially inaccessible areas of the aircraft such as cargo holds, main electrical load centres and anywhere wire is located in vulnerable sections of the aircraft.
  • The installation of Arc-Fault Circuit Interrupter (AFCI) circuit breakers. AFCB's detect low-level arc faults ('ticking') that traditional over current protective devices, such as the present generation of circuit breakers used in aircraft, cannot detect. 3

NOTES 1: At present no jet transport aircraft currently in production complies with the US Federal Aviation Regulation (FAR 25) pertaining to electrical system redundancy in the event of an electrical fire or arc tracking event. 4

              2: At present no jet transport has an effective strategy in place to deal with an inflight electrical fire as evidenced by the current Boeing B777 electrical fire drill which simply instructs aircrew to "Remove Power" from the affected circuit without providing them with any instructions as to how they should actually identify or determine the affected circuit(s) before removing power from them. Leaving power on the normal system wiring is to invite cascading systems malfunctions as wiring bundles flash-over. As a minimum requirement an aircraft electrical fire/smoke of unknown origin drill should instruct aircrew to immediately "power down" to Standby Electrical Power in order to protect the aircraft electrical system integrity. This action will markedly reduce the likelihood of trouble-shooting (and isolating) the fault and may in fact continue to power the faulty wiring, component or equipment. So what is actually required is a redundant fall-back system that can be selected at the first sign of trouble. This will avoid the irretrievable loss of vital systems and avoid any outbreak of a fire in hidden areas being stoked by power remaining on the wire. Because presently recommended pilot action is to land ASAP in the event of smoke, the provision of a fallback system may remove the urgency from the situation and minimize the possibility of an accident being generated by undue haste, particularly in bad weather/at night.  5


COMMENT: In Australia we have a non alcoholic drink called 'Claytons' TM , which is advertised by its manufacturers as "the drink you have when you're not having a drink". In the author's opinion, the Boeing Electrical Fire drill is a 'Claytons' emergency drill - the drill you have when you don't have a proper strategy to deal with the situation. Long drawn out protracted smoke checklists (preceded by air-conditioning smoke checklists) injected a significant delay into the fateful Swissair 111 scenario. The Chief MD-11 Test Pilot at McDonnell Douglas Long Beach (Tom Melody) published an aircrew bulletin wherein he admitted that the MD-11 smoke checklists would take a minimum of 35 minutes to complete and may not then resolve the situation. Note that during this period, because of the MD-11's electrical configuration, power would be being methodically removed from busses (per the drill) and then restored. This switching and the restoration of power by resetting tripped CB's is universally recommended against as it can induce a flash-over in defective wiring bundles. The persistent nature of electrical smells make it highly unlikely that crews will notice any difference as they switch between busses - but the chances of provoking a fire are greater.

Summary

As mentioned earlier, the current generation of jet aircraft such as the Boeing B777, Airbus and MD11 cannot be flown without electrics, unlike earlier generation jets such as the DC9. If a person had an electrical fire in their house, the first thing that person would do is remove the electrical supply to the house by pulling the fuses, but this option is not available with aircraft like the B777 and Airbus. The aircraft industry as a whole - that is aircraft manufacturers, government regulators, airlines and pilots - have failed to adequately address the problem of inflight electrical fire and are simply burying their collective heads in the sand about the issue. This is particularly apparent in the SFAR (Special Federal Airworthiness Reg) that addresses the considerations for ETOPS (extended range operations over oceanic and desolate unlandable areas. The concept of a Virgin Electrical Bus is a significant step towards providing a solution to this complex problem and affording a level of redundancy that presently does not exist.

FOOTNOTES

1. Swissair Flt 111 experienced a smoke-in-the-cockpit emergency shortly after reaching top of climb out of New York on the night of 2nd September 1998. The aircrew commenced a diversion to Halifax, but 20 minutes later the aircraft crashed into the sea off the Nova Scotia coast whilst the crew were still attempting to carry out the lengthy trouble-shooting checklist. The electrical fire is thought to have been associated with the unapproved installation of an in-flight entertainment system (which incidentally had no ON/OFF switch installed). There is some evidence that Kapton wire in the aircraft may have caused or contributed to this crash. It would appear the lengthy trouble-shooting checklist also contributed to the disaster because electrical power was not removed from the electrical circuits involved. MD11 aircraft cannot be flown without electricity, so the removal of all electricity from the aircraft was not an option available to the SR111 aircrew. For more information about this incident see:

NOTE: It is the opinion of many aviation safety experts (including the author) that the SR111 crash should prove to be a watershed event because it will lead to far reaching changes within the industry with regard to the design of aircraft electrical systems and wire, as well as numerous other aspects of aviation safety.

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2. For more on KAPTON wire see:

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3. ARC-FAULT CIRCUIT INTERRUPTER (AFCI) circuit breakers. There are significant design constraints facing the concept of AFCI circuit breakers for aircraft, not in the least being the likelihood of high frequency attenuation associated with the long lengths of wire in aircraft.  Because of these design problems, most aviation electrical experts don't expect to see reliable AFCI circuit breakers in aircraft prior to 2005. However some airlines are trialing them at present (Feb 04) in non-vital areas.

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4.FAR 25 states in part:

  • Sec. 25.1351 General.
    (2)     No failure or malfunction of any power source can create a hazard or impair the ability of remaining sources to supply essential loads.
    COMMENT:     Aircraft wires associated with a plethora of diverse systems and whose power source and voltage are often different are currently all bundled together in massive wire looms. (bundles often involving hundreds of wires) An explosive arc-tracking event invariably severely damages surrounding wires, taking out unrelated systems and causing further electrical fires and explosions. TWA Flight 800 which exploded over New York on 17 July 1996 is thought to have been caused by low voltage fuel tank quantity wires shorting out to adjacent high voltage wires triggering an explosion in the centre fuel tank and destroying the aircraft.

     
  • Sec. 25.1351 General.
    (6) (d)     Operation without normal electrical power. ...
    ... Parts of the electrical system may remain on if--
    (1) A single malfunction, including a wire bundle or junction box fire, cannot result in loss of both the part turned off and the part turned on; and
    (2) The parts turned on are electrically and mechanically isolated from the parts turned off.

         COMMENT: As above, this requirement cannot be met due to the fact that the wires from various systems (including emergency power items) are usually bundled together in massive wire looms.

     
  • Sec. 25.1353 Electrical equipment and installations.
    (a) Electrical equipment, controls, and wiring must be installed so that operation of any one unit or system of units will not adversely affect the simultaneous operation of any other electrical unit or system essential to the safe operation.
    (b) Cables must be grouped, routed, and spaced so that damage to essential circuits will be minimized if there are faults in heavy current-carrying cables.

         COMMENT: As above, this requirement cannot be met due to the fact that the wires from various systems (including emergency power items) are usually bundled together in massive wire looms.

     
  • Sec. 25.1357 Circuit protective devices.
    (a) Automatic protective devices must be used to minimize distress to the electrical system and hazard to the airplane in the event of wiring faults or serious malfunction of the system or connected equipment.

    COMMENT: The current generation of bi-metallic circuit breaker used in aircraft have some serious flaws rendering them inadequate for the task of being aircraft "circuit protective devices" as stipulated in FAR 25.1357. Bi-metallic circuit breakers cannot detect the micro discharges of electrical current through hairline cracks in the insulation material of aircraft wire. (a condition known as 'ticking') Ticking faults can eventually lead to a major 'flashover' event in which the insulation material of the wire catastrophically fails leading to a very high current discharge of energy through the wire to the surrounding aircraft structure. (i.e. a short circuit) In the case of KAPTON wire, this flashover event is usually explosive, resulting in severe damage to the surrounding wires with potentially catastrophic results. (e.g. SR111 crash) The situation can be further exacerbated by the fact that bi-metallic circuit breakers can actually 'weld' themselves closed due to the very high heat generated in the circuit breaker during a flash-over event. Arc-Fault Circuit Interrupter (AFCI) circuit breakers may be the answer to these problems, however see     footnote 3 above.

Source: FAR 25 - Electrical Systems and Equipment: http://www.faa.gov/avr/AFS/FARS/far-25.txt

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5 B777 ELECTRICAL FIRE DRILL

SMOKE/FUMES/FIRE ELEC
Condition: A concentration of electrical smoke/fumes or fire is identified.

OXYGEN MASKS AND SMOKE GOGGLES ON
(If required)

CREW COMMUNICATIONS ESTABLISH
(If required)

If smoke/fumes or fire source can be determined:
ELECTRICAL POWER (affected equipment) REMOVE

If smoke/fumes or fire is persistent:
Plan to land at the nearest suitable airport.

Source: Boeing B777 Flight Manual

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Copyright © Alex Paterson (2000)

 

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