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Boeing 747-436, G-BNLC
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AAIB Bulletin No: 9/2003
Ref: EW/G2003/04/27
Category: 1.1
INCIDENT
Aircraft Type and Registration:
Boeing 747-436, G-BNLC
No & Type of Engines:
4 Rolls-Royce RB211-524G2-19 turbofan engines
Year of Manufacture:
1989
Date & Time (UTC):
20 April 2003 at 1200 hrs
Location:
Airborne, Riga FIR, Latvia
Type of Flight:
Public Transport (Passenger)
Persons on Board:
Crew - 18
Passengers - 307
Injuries:
Crew - None
Passengers - None
Nature of Damage:
Overheating damage
to cockpit door lock solenoid assembly
Commander's Licence:
Airline Transport Pilot's Licence
Commander's Age:
47 years
Commander's Flying Experience:
12,000 hours (of which 9,000 were on type)
Last 90 days - 200 hours
Last 28 days - 60 hours
Information Source:
Aircraft Accident Report Form submitted by the pilot and subsequent
enquires by the AAIB
Synopsis
During cruise the crew became aware of fumes and smoke in the cockpit.
The crew went onto oxygen, declared a MAYDAY and diverted to Riga,
Latvia. Subsequent investigation revealed that the smoke and fumes were
as a result of an overheated cockpit door lock solenoid, which had
failed due to spring clip being incorrectly installed during the
manufacture of the unit.
History of Flight
G-BNLC had departed Calcutta and was in the cruise at FL360 in the Riga
FIR. A crewmember, who had previously left the cockpit during the
flight, requested access back to the cockpit so that he could begin his
rest period. When he requested entry, this was initially denied by the
flight crew. However, once they verified that he was a crewmember they
electrically unlocked the cockpit door. The crewmember initially had
difficulty opening the door but, after a few attempts, it was given a
'bang' after which it opened. Once the door was closed again it was
found that it would not electrically lock and it was at this point the
crew also noticed that the LOCK FAIL light on the centre pedestal was
illuminated. The crew consulted the minimum equipment list (MEL), as
there were no other published procedures for dealing with a continuous
LOCK FAIL indication.
The technical manual carried on the aircraft, however, did give a
description of what the LOCK FAIL light indicates and, in addition, a
flight crew notice (FCN), issued in 2002, detailed
(security?)
procedures for dealing with door unserviceabilities. Therefore, the
crew manually locked the door as this was the required action given in
both the MEL and the FCN. About 15 to 20 minutes later, the handling
first officer (FO1) reported that he could smell fumes in the cockpit
that were similar, in his opinion, to an electrical burning smell. The
commander agreed that he could also smell the fumes, but thought that
the source was from the galley. He consulted and confirmed with the
upper deck purser that the upper galley ovens had just been switched on
and so he thought that the smell would soon dissipate.
However, the smell continued.
The commander then consulted the Cabin Service Director (CSD) as to
whether there were any unusual smells in the cabin. The CSD stated that
there was an unusual smell at seat 5K in the first class cabin, and that
the power to the seat had been isolated.
As a further precaution the commander
requested that the in flight entertainment system be switched off.
In addition, the commander selected off the air conditioning
re-circulation fans. Following this, the upper deck purser and the CSD
both reported to the flight deck but as they did so they stated that the
smell was stronger on the flight deck than in the cabin.
<<Sounds to be a very similar development to
an SR-111 scenario>>
At this point the commander considered that should the source of the
smell not be isolated a diversion might be necessary. He had identified
Riga Airport as a suitable airfield which he could see out of his
right hand window. << (from
cruise altitude)>>
<<Note: the
advantages of a 3 man crew ==>>
The FO1 donned his oxygen mask whilst the commander left his seat to
both check if the source of this smell was on the flight deck and to
awaken the resting FO2. As FO2 left the rest area, he immediately
became aware of the fumes. The commander returned to his seat, FO1 and
FO2 swapped positions with FO2 taking over as the handling pilot. The
flight crew donned their oxygen masks and were briefed by the commander
on the intention to divert. A MAYDAY was declared and air
traffic control (ATC) was told that the aircraft would divert to Riga.
The crew completed the initial Quick Reference Handbook (QRH) actions
for an indication of SMOKE/FUMES and, at this time, the commander found
communication between the crew, the cabin staff and ATC was difficult.
The passengers were briefed on the problem and were informed that they
were diverting. The commander then carried out a radar vectored
approach to Riga but, due to misting in
his oxygen mask visor, he removed his mask for better visibility during
the final approach. The other crewmembers were asked to monitor
his performance during the approach and landing.
The landing was without incident and the aircraft was parked away from
the terminal building with the emergency services in attendance. The
passengers disembarked normally via steps provided by airport personnel.
Once the aircraft had been fully shut down, the flight crew became
aware that the flight deck door surround
was hot and that there was a strong smell emanating from the cockpit
door lock striker assembly in the doorframe.
Subsequent medical examination of FO2 revealed that he had raised levels
of carbon monoxide in his blood.
Aircraft Examination
At Riga, examination of the aircraft revealed that the cockpit door lock
mechanism had overheated with smoke issuing from the cut-out for the
door-striker. The source of the overheating was from the cockpit door
lock solenoid and this was subsequently electrically isolated before the
aircraft was ferried back to the UK.
Cockpit Door Lock Description
The cockpit door lock on G-BNLC was a
recent modification in February 2003,
as a result of increased cockpit access security required by the FAA
following the terrorist incidents in September 2001. This was a 'phase
2' system having replaced the initial intermediate installation; 'phase
1'.
The 'phase 2' door lock system consists of a catch attached to the
mid-position of the cockpit door. When the door is closed, a
spring-loaded striker assembly in the doorframe retains the catch in
place, holding the door closed. The door is locked in place
electrically by a solenoid, which forms part of the strike assembly.
The solenoid consists of a shaft that moves upward when an electrical
current is applied. The solenoid shaft is attached, via a hexagonal nut
(to allow for adjustment) to a locking pin. When the solenoid shaft
moves upward, the locking pin is forced up behind the striker which
holds it in place, thus locking the door. When the solenoid shaft has
moved upward to its full extent, a micro switch within the solenoid
operates and, to prevent overheating, reduces the electrical current to
a level low enough to just hold the solenoid and the door in the locked
position. At manufacture, the hexagonal nut is used to adjust the
locking pin and is retained by the use of a spring clip.
The door lock solenoid and the cockpit door
system are protected by a 2.5 A circuit breaker.
The door lock system is controlled from a door switch module located on
the centre pedestal. Should the door lock system fail, such as a
failure of the door lock solenoid to fully lock when required, an amber
LOCK FAIL light will illuminate on this module. Although the light is
an amber caution warning, this is not linked to the aircraft's master
warning system and therefore neither gives an annunciation on the glare
shield nor a caution message on the central display of the EICAS.
It is possible for the crew to unlock the door, in the event of a
failure of the electrical locking system, as a manual override of the
door latch is fitted, which is only
operable from the inside of the cockpit.
Detailed Aircraft Examination
The cockpit door lock striker assembly was removed from the doorframe
and this revealed that the overheating was limited to the solenoid. No
other damage to the surrounding area had occurred. Further
investigation revealed that the spring clip, which retains the hexagonal
nut, had rotated in such a way as to prevent the solenoid from fully
retracting when electrical power was removed to unlock the door. Thus,
with the solenoid shaft semi-retracted, the lock pin was not fully
retracted from the spring-loaded striker. With the lock pin in this
position the spring-loaded strike had become jammed on the top of the
lock pin and this was confirmed by associated witness marks. This
probably occurred when the door was electrically unlocked to allow the
crewmember access to the cockpit and is consistent with the difficulty
he had in opening the door. When the door was 'banged' the striker was
forced over the top of the lock pin, jamming it in position.
With the spring-loaded striker jammed on the lock pin, the solenoid
would not have been able to extend upwards when the door was
subsequently electrically locked. This would explain the problems the
crew had with locking the door after the crewmember had gained entry.
Also, this would explain the illumination of the LOCK FAIL light, as the
internal lock solenoid micro-switch, which signals both a locked door
and a reduction in the electrical current to the solenoid, had remained
open. This meant that full electrical current would have been
flowing continuously.
As the solenoid is not rated for this
condition, it will overheat with the resulting smoke and fumes
experienced by the crew.
[but doesn't explain why the CB did not trip]
Safety action
The aircraft manufacturer later confirmed that the hexagonal nut spring
clip had been able to rotate due to improper installation during the
original manufacture of the unit. Due to this finding, the door striker
assembly manufacturer issued a Service Bulletin (SB) to inspect the door
lock assemblies for correct installation. In addition, the aircraft
manufacturer has informed all operators of the incident and the
availability of the SB. The operator has already undertaken an
inspection of the doors of all their aircraft fitted with this system.
There were no procedures in place for
the crew to cover the instance of a continuous LOCK FAIL illumination,
except for the information contained in the MEL and FCN,
and therefore there were
no specific instructions to the crew to electrically isolate the system.
The operator has since changed their flight crew procedures such that
crews are instructed to electrically isolate the system if the LOCK FAIL
light remains illuminated, especially if a door lock solenoid overheat
failure is suspected. Secondary security procedures are then invoked.
The aircraft manufacturer is also amending their documentation to
clarify the actions to be taken when a LOCK FAIL light illuminates.
But what about changing the rating
of the CB?
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