CAPABILITIES FOR THE REMAINDER OF THE FLIGHT
steps to remove power from affected equipment must ensure that sufficient
system capability remains to accommodate adverse weather, a replanned
route, and an approach into an unfamiliar airport. In-service data
show that inordinate depowering 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
depowering 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 depowering 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 pressurized area of the airplane. From this
standpoint, using the overhead switch to depower 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 depower all potential
sources of unknown smoke without compromising necessary systems.
The key to depowering 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 depowered 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
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.