Published: April 2001
Engineering design by airplane manufacturers, oversight by regulators, and maintenance practices by operators combine to minimise occurrences of smoke, fumes, and fire in the pressurised areas of airplanes. When smoke does occur, timely and appropriate action by the flight and cabin crews is imperative. Boeing has analysed in-service smoke, fumes, and fire events and reviewed airplane systems and crew procedures for its commercial airplane models.
An in-flight fire or smoke event is a time-critical situation that demands immediate action by the flight and cabin crews. Cigarettes aside, any smoke in an airplane is not normal. Crew response must be timely and use available airplane controls and non-normal procedures.
To help ensure that appropriate steps are taken, the following issues need to be understood:
- Operational consequences and safety risks of smoke events.
- Analysis of past smoke events and review of crew procedures.
- Recommended crew action for known and unknown smoke sources.
- Capabilities for the remainder of the flight.
1. OPERATIONAL CONSEQUENCES AND SAFETY RISKS OF SMOKE EVENTS
Although most smoke events in the pressurised area of an airplane are resolved and rarely affect continued safe flight, landing, or egress, smoke is always a significant issue with operational consequences. These consequences include flight cancellations, flight schedule disruptions, air turn-backs, airplane diversions, declared emergencies, airport emergency equipment responses, airplane evacuations, accommodations for displaced passengers, diminished goodwill, and extensive unscheduled maintenance following non-normal procedures such as overweight landing inspection, recharging of oxygen, and repacking of escape slides.
Direct crew response to smoke and fumes originating from readily accessible equipment, referred to as known smoke, is key to minimising operational consequences. Timely and prudent crew response to smoke events of undetermined origin, or unknown smoke, minimises risks during the remaining flight, landing, and egress.
Based on past smoke events, Boeing and other air transport industry leaders are pursuing initiatives to further reduce the likelihood of in-flight smoke. In addition to enhancements to airplane design and maintenance (see "Aging Airplane Systems Investigation," Aero no. 7, July 1999), these initiatives include improvements to the procedures used by the flight and cabin crews during a smoke event in the pressurised area of the airplane.
2. ANALYSIS OF PAST SMOKE EVENTS AND REVIEW OF CREW PROCEDURES
Boeing performed an analysis of reported in-service events that involved smoke, fumes, fire, and overheating in the pressurised areas of its airplanes between November 1992 and June 2000. Data were compiled for each model and included the following: the area affected in the pressurised area of the airplane, the smoke source perceived by the flight crew, the smoke source identified by the maintenance crew, the category of the smoke source, the airplane system or equipment involved, the means of detection (typically sight or smell by passengers or crew), and the effect on flight completion. (Note: The term smoke in the preceding list and in the remainder of this article refers to odours, smells, fumes, or overheating as well as visible smoke.)
The smoke events under study were categorised into three classes: air conditioning, electrical, and material. Air-conditioning smoke events were cases in which incoming bleed air was contaminated, perhaps from engine oil or contaminated outside air. Electrical events were cases in which electrically powered equipment overheated or emitted smoke or fumes. Material events involved material that gave off smoke or fumes such as food burning in an oven, lavatory waste ignited by a discarded cigarette, or spilled chemicals in the cargo compartment.
Figure 1 depicts a summary profile of air-conditioning, electrical, and material smoke events for each airplane model included in the study. This format enables comparison across airplane models of the three major smoke source categories. For each model, the number of events in each source category was divided by the total number of smoke events for that model, yielding the percentage contributions depicted in the profile. (Note: The three categories for each model may not sum to 100 percent because of insufficient information available to categorise an event.) The models in figure 1 are listed in order of airplane complexity, starting with the most complex on the left. Larger airplanes with more complex systems show a predominance of smoke events of electrical origin, compared with air-conditioning and material smoke events.
For each airplane model, the air-conditioning, electrical, and material events were subdivided by airplane system. Figure 2 illustrates such a detailed categorisation of smoke event sources for a representative model. The subcategories within the electrical category include systems or functions such as environmental control, electrical power, galleys, and flight deck equipment. Presenting the smoke sources in percentages by airplane system or function allows comparison of multiple models with different fleet sizes, ages, and missions.
Data also were collected on how the crews perceived the in-flight smoke events on all models. The data were grouped in a structure similar to the flight crew Quick Reference Handbook (QRH) produced by airplane manufacturers and operators. Figure 3 shows such a portrayal for a representative model. Most smoke events occurred with the flight crew on board. For many in-flight events, flight crews took action consistent with having identified the smoke source, such as removing electrical power to (i.e., de-powering) that equipment. There was a significant number of events in which crew actions suggest that the smoke source could not be identified while in flight. For smoke events in which the flight crew could not determine the smoke source, most were subsequently determined by maintenance crews to be of electrical origin.
3. RECOMMENDED CREW ACTION FOR KNOWN AND UNKNOWN SMOKE SOURCES
The Boeing QRH includes procedural steps for smoke, fumes, fire of air-conditioning and electrical origin, and smoke removal. When a flight crew has determined that smoke is of air-conditioning origin, the Boeing QRH procedure is to isolate the air source, halting the introduction of contaminated air into the pressurised area of the airplane. An example of air-conditioning smoke is from engine oil, followed by abnormal engine parameters and odour in the cabin and flight deck. Once the crew has isolated the incoming engine bleed air, continuing fresh air from another source should quickly improve cabin air quality.
When a flight crew determines smoke is of electrical origin, the Boeing QRH procedure is to de-power the affected equipment. For example, if a flight crew sees smoke from a window-heating element, appropriate action would be to switch off that electrical equipment. An example of known smoke in the cabin would be a flight attendant seeing and smelling smoke from a coffee maker; after turning off electrical power to that galley, the smoke stops and subsequent surface temperatures are normal. The key to properly handling a known smoke event is for the crew to be confident of both the smoke source and the effectiveness of removing electrical power.
Known smoke sources.
Many smoke events involve smoke or fumes produced by equipment readily accessible to the crew. Often, the event source can be identified by direct observation, such as seeing smoke exiting a piece of equipment, tracing a smell to its strongest location, or feeling an unusually warm surface.
For a known smoke event, confirming that the situation has been resolved is as important as identifying the source. The smoke or fumes must dissipate and any overheating condition must improve for the crew to be confident the situation is under control. Only if the crew can confidently identify the smoke source and confidently ascertain that the condition is under control should continuation of the flight be considered. Hand-held extinguishers ought to be at the ready, as the crew continues monitoring the equipment during the remainder of the flight.
Factors to evaluate in deciding include the level of confidence in identifying the smoke source, success in extinguishing the source, functionality of the remaining systems, success in removing cabin smoke, passenger distress, and position of the airplane along the intended route. Any combination of these factors may make a diversion or turn-back the appropriate choice.
Completing a planned flight has its advantages given the significant operational costs of substitute equipment, schedule disruption, potential passenger compensation, and diminished goodwill. The best prospect for minimum disruption from a smoke event comes from crew training in responding to smoke, crew familiarity with smoke-clearing procedures, and direct power control to cabin amenities (e.g., an electrical power cutoff switch at each galley location). If the crew cannot confirm that a persistent onboard smoke or fire situation is completely resolved, however, Boeing recommends the earliest possible descent, landing, and evacuation of the airplane.
Unknown smoke sources.
A crew may not be able to identify a smoke source because of the location of the failed equipment or because of air circulation throughout the pressurised cabin. Unknown smoke sources include environmental control systems, equipment cooling fans, door heaters, plumbing heaters, avionics equipment, fluorescent lights, and wiring faults. The serious consequences of compromised structural integrity, system function, or survivable environment warrant timely and prudent action by the crew. Review of historical data on the rare fire events that resulted in hull loss indicates that the time from first indication of smoke to an out-of-control situation may be very short — a matter of minutes. For this reason, flight crew actions when responding to unknown smoke must be timely and appropriate.
QRH procedural steps for addressing an undetermined electrical smoke source call for the removal of electrical power for specific systems not necessary for safe flight, landing, and egress. This accounts for the majority of systems with a significant history of in-service smoke events. Also, as directed by the Boeing QRH non-normal checklist, the crew should plan to land at the nearest suitable airport. During the remainder of the flight, the crew should be alert to any new signs that suggest the smoke source and remain mindful of operational functions needed to accomplish the diversion.
Many unknown smoke situations are later attributed to electrical sources, substantiating the positive step of de-powering specific equipment not necessary for the remaining flight, landing, and egress. Flight-critical systems do not have a significant smoke-event history.
4. CAPABILITIES FOR THE REMAINDER OF THE FLIGHT
QRH procedural steps to remove power from affected equipment must ensure that sufficient system capability remains to accommodate adverse weather, a re-planned route, and an approach into an unfamiliar airport. In-service data show that inordinate de-powering of airplane systems beyond QRH procedures is not likely to be of benefit in an unknown smoke situation. Further, such action would significantly reduce airplane capabilities for the remainder of the flight.
During the study, several de-powering strategies beyond current procedures were considered but ultimately not incorporated into the Boeing QRH non-normal checklists based on a risk-benefit evaluation. The elements of continued safe flight and landing were determined according to four safety requirements: controlled flight path, controlled airplane energy, navigation, and survivable environment. Conditions during the remainder of the flight could necessitate the availability of flight management system navigation, autopilot, multiple communication channels, first officer's displays, smoke detection, fire suppression, cabin lighting, and electrical power for removing smoke.
Exterior lighting illustrates the important difference between a prudent crew response and an inordinate de-powering of airplane systems during an unknown smoke event. Equipment used for red anti-collision strobes includes high-energy components, such as a high-intensity flasher, and is an occasional source of smoke in the pressurised area of the airplane. From this standpoint, using the overhead switch to de-power red anti-collision strobes may be beneficial during an unknown smoke event. Turning off all exterior lighting, however, would be an overreaction that would increase the risk of traffic conflict without commensurate likelihood of addressing the smoke source.
Without complicated troubleshooting-type procedures, it is a practical impossibility to de-power all potential sources of unknown smoke without compromising necessary systems. The key to de-powering potential unknown smoke sources while protecting necessary airplane functions involves balancing a series of risk assessments. Because the QRH must facilitate timely and prudent crew action appropriate for a broad range of scenarios, the QRH procedures cannot resort to a severely de-powered electrical configuration. Boeing QRH procedures are developed with the understanding that, at a flight crew's discretion, additional action may be taken that is deemed necessary to ensure safe flight.
If a flight crew considers action beyond the QRH procedures, the action must be based on the particular situation and knowledge of airplane system operation. Procedural alternatives that may be reasonable near a familiar airport under visual meteorological conditions may not be appropriate in adverse weather or unfamiliar surroundings with a compromised airplane. The crew may also have additional flight deck effects or information beyond those explicitly identified in the QRH (e.g., tripped circuit breakers, synoptic information, or reports from cabin crew) that may assist in identifying the smoke source.
A flight crew in an extreme situation will benefit from airplane system knowledge that would be inappropriate to detail in time-critical procedures. For example, on most Boeing-designed two-engine airplanes, the right electrical bus powers a higher proportion of nonessential equipment, while the left electrical bus powers the higher proportion of flight-critical equipment.
The best response to an event of unknown smoke combines use of prudent QRH non-normal checklists and flight crew discretion based on the particular situation and a thorough knowledge of airplane systems.
Summary
|
TIPS ON MINIMISING SMOKE EVENTS
The following tips are based on the review and analysis of in-flight smoke events on Boeing airplanes between November 1992 and June 2000:
-
Although not a serious risk for propagating fire, several events occurring immediately before or after airplane departure were attributed to engine or auxiliary power unit (APU) maintenance activity during the previous ground leg. Most operators have ground crew procedures for engine or APU runs following maintenance. For an operator with concerns in this area, a review of ground procedures that require engine or APU run may be appropriate.
-
Some known smoke events are directly preventable. Paper may come into contact with hot lighting, either in the cabin or crew rest areas. Food may be left in an oven or a coffeepot heated while empty.
-
Smoke or actual fire events have been initiated by repeated circuit breaker resets during ground troubleshooting. Even when performed on the ground, circuit breaker resets should be performed cautiously. Important considerations are the number of reset attempts, cooling time between reset attempts, and the stationing of maintenance crew monitoring for unusual sounds or smell.
-
A flight crew may be able to identify unknown smoke as air-conditioning smoke based on subsequent indication. In an air-conditioning smoke event caused by leaking engine oil, the first symptom noticed by the crew may be a burning odour of unknown origin. Subsequent engine indications might clarify an abnormal engine situation, and the corresponding bleed air source can be isolated.
JOHN MORTON
PRINCIPAL ENGINEER
SYSTEMS ENGINEERING
BOEING COMMERCIAL AIRPLANES GROUP
ROGER NICHOLSON
ASSOCIATE TECHNICAL FELLOW
SERVICE ENGINEERING
BOEING COMMERCIAL AIRPLANES GROUP
BILL ROBERSON
DIRECTOR EXECUTIVE
FLIGHT OPERATIONS
BOEING COMMERCIAL AIRPLANES GROUP
| Top of page | Boeing Home | Boeing Commercial | Contact Aero |
Aero Copyright © 2003 The Boeing Company. All rights reserved.