DEPARTMENT
OF THE AIR FORCE
HEADQUARTERS
UNITED STATES AIR FORCE
WASHINGTON
Office
of The Inspector General USAF
Norton Air Force Base
San Bernardino, California
19 September 1952
Report
of Special Investigation of Major Aircraft Accident
Involving B-36D, SN 49-2661, at San Diego Bay,
San Diego, California, on 5 August 1952
THE
ACCIDENT
1. B-36D aircraft, SN 49-2661, on bailment to Consolidated
Vultee Aircraft Corporation, (Convair), San Diego,
California, crashed into San Diego Bay at 1430 PDT, 5
August 1952 while on a normal shakedown flight following
completion of "San-San" project modification. The aircraft
was destroyed by impact and explosion. Four of the eight
crewmembers received minor injuries, two were uninjured,
and two are missing.
CONCLUSIONS
2. It is concluded that:
a. The primary cause of the accident was fire in the
vicinity of No. 5 engine (see pars 10 and 12).
b. The probable causes of the fire were:
(1) A malfunction of the No. 5 alternator and/or alternator
constant speed drive (CSD) resulting in extreme heat and
ignition of a magnesium fire (see par 20).
(2) A fuel leak in the vicinity of No. 5 engine (see pars
17, 18 and 19).
(3) Material failure of the No. 5 engine turbo supercharger
or exhaust systems, allowing residual oil to become ignited
by exhaust flame (see par 14).
c. The use of magnesium alloy materials adjacent to
potential fire areas in B-36 aircraft constitutes a hazard
(see par 12).
d. The Edison fire detection system is not adequate (see
par 21).
e. The right scanner detected smoke coming from the No. 5
engine, but did not report the observation in accordance
with the procedure outlined in T.O. 01-5EUC-1 dated 15
March 1952. As a result the fire extinguishing system was
not activated (see par 22).
f. The flight test crew did not comply with T.O.
01-5EU-355, dated 21 December 1951, in the operation of the
electrical system during the flight (see par 20).
g. The emergency fuel shut-off procedure outlined in T.O.
01-5EUC-1, dated 15 March 1952, is inadequate for combating
engine and wing fires (see par 17).
h. The issuance of T.O. 01-5EU-352, dated 31 January 1952,
subject: "Sealing of Fuel System Hose Connections - All
Series RB-36 and B-36 Aircraft," as a red diagonal instead
of a red cross compliance constitutes a serious flight
hazard in view of the critical nature of fuel leaks in B-36
aircraft (see par 19).
i. The placard instructions on the flight engineer's panel
outlining the procedure to follow in combating engine fires
are not in accordance with instructions contained in
applicable B-36 technical orders (see par 22).
j. The absence of the right scanner from his duty for
approximately 15 minutes delayed detection of the emergency
(see par 24).
k. Members of the contractor flight crew were not wearing
their life vests at the time the emergency occurred (see
par 25).
RECOMMENDATIONS
3. IT IS RECOMMENDED THAT THE COMMANDING GENERAL, AIR
RESEARCH AND DEVELOPMENT COMMAND:
a. Examine possible means of reducing the hazard resulting
from the use of magnesium alloys adjacent to potential fire
areas in future USAF aircraft and revise the HIAD
accordingly (see par 12).
b. Evaluate fire detection systems installed in all USAF
aircraft and develop reliable devices which provide
positive warning of wing or engine fires (see par 21).
4. IT IS RECOMMENDED THAT THE COMMANDING GENERAL, AIR
MATERIEL COMMAND:
a. Require that contractor flight test crews are familiar
with and comply with applicable technical orders for
reporting observations of an emergency nature, for engine
shut-down emergency procedures and for combating engine and
wing fires in USAF aircraft (see par 22).
b. Require that contractor flight test crews comply with
T.O. 01-5EU-355, subject, "Operation of the Electrical
System of B-36 Series Aircraft in which Exciter Protection
Relays are not Installed" (see par 20).
c. Revise B-36 Operating Instructions to insure that the
emergency fuel shut-off procedure will isolate engine or
wing fires from the fuel manifold system (see par 17).
d. Require compliance with technical orders involving
safety of flight items prior to the next flight of the
aircraft involved after receipt of kits (see par 19).
e. Insure that the engine shut-down procedure placard in
B-36 and RB-36 series aircraft is in agreement with
pertinent technical orders (see par 22).
f. Revise B-36 Operating Instructions to require that left
and right scanners' stations are manned at all times during
flight (see par 24).
g. Take action to insure that contractor flight test crews
wear life vests when engaged in over-water flights in USAF
aircraft (see par 25).
ACTION
TAKEN
5. The recommendations contained in paragraph 3 have been
referred by letter to the Commanding General, Air Research
and Development Command, for action and reply.
6. The recommendations contained in paragraph 4 have been
referred by letter to the Commanding General, Air Materiel
Command, for action and reply.
HISTORY
OF FLIGHT
7. B-36D, SN 49-2661, took off from Lindbergh Field at 1039
PDT, 5 August 1952 on a routine shakedown flight following
completion of "San-San" modification. The aircraft was
operated by a Convair civilian flight test crew. After
approximately three hours of flight, a decent was made to
4,000 feet where two normal landing gear extensions and
retractions were made. The crew then prepared to accomplish
a manual emergency gear extension.
8. The right scanner, a qualified Convair B-36 flight
engineer, and the radar technician were selected to
manually extend the gear. The right scanner experienced no
difficulty in dropping the right gear. However, the radar
technician, having less experience in this operation, had
difficulty in lowering the left gear. He crawled out of the
left wheelwell area and requested the right scanner to
accomplish the left gear extension also. The radar
technician then proceeded from the left wheelwell to the
right wing root area adjacent to the right wheelwell. He
announced over the interphone that the right scanner was
also lowering the left gear. Shortly thereafter, the right
scanner emerged from the left wheelwell area stating that
the gear had been extended. The radar technician, in the
meantime, had detected gasoline fumes coming from the right
wing crawlway access port. When the right scanner returned
from the left wheelwell area, he was asked by the radar
technician to confirm and investigate the fuel fumes noted.
He did this but considered the presence of fumes of no
consequence. He explained that the fumes were probably
coming from the right wing auxiliary and No. 4 tank vent
outlets that are below and aft of the wheelwell.
9. Upon completion of these checks, the aircraft was
scheduled to land. Immediately upon return to his station,
the right scanner detected smoke coming from the No. 5
engine and advised the engineer on station at the panel to
feather No. 5 engine. He then instructed the flight
engineer to shut off the oil and fuel to this engine. The
flight engineer complied with these instructions. Seconds
later there was an explosion in the right wing. The
Aircraft Commander, in the copilot's seat, glanced out the
window at the right wing area and directed that the
aircraft be abandoned. This order was heard and obeyed by
all crewmembers; however, the Aircraft Commander did not
successfully evacuate the aircraft. The B-36 crashed into
the sea about three and one-half miles from shore and
exploded on impact. The wreckage sank in
approximately XXX
feet of
water. Six crewmembers were rescued. Two crewmembers are
missing.
INVESTIGATION
AND ANALYSIS
10. Interrogation of witnesses disclosed that the aircraft
was flying straight and level, with the landing gear
extended, at an altitude of approximately 4,000 feet MSL
when light grey smoke was first observed coming from the
right wing area. At approximately 1426 PDT, an explosion
occurred in the vicinity of No. 5 engine at which time
black smoke and fire were observed. Immediately thereafter
No. 5 engine fell from the aircraft, and intensity of the
fire increased. As the aircraft lost altitude, witnesses
observed seven crewmembers parachute from the aircraft.
Small portions of the aircraft were seen to fall free as it
continued its descent. The aircraft turned slowly to the
right approximately 200 degrees from its original heading
and crashed into the sea in a steep dive. At the time of
initial impact, there was an explosion followed by fire
that covered an area approximately 200 yards in diameter.
11. The No. 6 engine, two sections of the outer right wing
panel between No. 6 engine and the jet pod, a portion of
the No. 5 fuel tank, a part of an outboard engine mount
from No. 4 engine, small pieces of metal, some electrical
wiring, and a portion of one bomb-bay door were recovered.
However, examination of these parts failed to reveal the
cause of the accident.
12. Magnesium alloy is used in the fabrication of casings
around and aft of the engine cooling fan, induction system
air duct, alternator housing (bullet), major portions of
the nacelle skin, aileron skin, engine cooling air duct
(tunnel), wing leading and trailing edge, and integral
portions of the alternator. The advisability of using
magnesium alloy in the fabrication of those components of
the B-36 wing and engine nacelle adjacent to potential fire
areas is questionable. In this accident it is suspected
that the origin of the fire was in the No. 5 engine
nacelle. It seems improbable that the fire started in any
other portion of the wing, since the right wing appeared to
be normal during the time the landing gear was being
manually extended by the crew. At the completion of this
operation, the crew noticed gasoline fumes and again
checked the interior of the right wing from the wing
crawlway access port. A this time, no smoke or fire was
observed. The absence of any smoke or fire at this time and
the fact that the flight engineer did not receive a fire
warning light indication, together with the fact that there
was no fuel, oil or other instrument variation, suggests
that the fire originated from a magnesium source. This is
substantiated by the severity of the fire, the heavy white
smoke, and by the Aircraft Commander's immediate decision
to abandon the aircraft. The intensity of the fire would be
substantially increased when fuel lines severed either by
the magnesium fire itself, by the reported initial
explosion, or by the No. 5 engine falling from the
aircraft.
13. The surviving crewmembers stated that immediately prior
to the emergency, all reciprocating engines were operating
normally. Power settings were 35 inches manifold pressure,
2200 RPM with turbo master control at zero. The mixture
controls were normal. Air plugs of jet engines Nos. 2 and 3
had just been activated to the open position in preparation
for airstart. Earlier in the flight, the radar technician
was unable to check the optical sight on the
bombing-navigational equipment because of aircraft
vibration. At that time, propellers were being operated at
1550 RPM; however, a subsequent feathering check of all
propellers indicated that vibration was not due to any
particular engine or propeller. It was the opinion of the
radar technician that most of the vibration was caused by
No. 3 engine. With the exception of the left scanner, who
visually observed some vibration on No. 3 engine, the
remainder of the crew did not detect the vibration or
consider it of any consequence.
14. The oil systems of the reciprocating engines operated
normally before the emergency. There was no instrument
indication of an oil leak. The first reported light grey
smoke could have been caused by oil leaking on the hot
exhaust manifold of No. 5 engine or by an oil leak in the
No. 5 engine turbo supercharger. A review of Unsatisfactory
Reports on B-36 aircraft, wherein fire was a possibility,
revealed there were a total of 163 reports submitted during
the period January through July 1952, concerning electrical
components which would have caused or contributed to fire.
This study indicates that materiel failure of the turbo
supercharger could have been a possibility. There were 507
Unsatisfactory Reports submitted during the same period on
specific exhaust system items which may have caused or
contributed to fire. However, since a magnesium fire was
observed in the vicinity of No. 5 engine, it appears that
the fire originated from a source other than the
supercharger oil.
15. The hydraulic system was normal, except that sluggish
operation was experienced with the nosewheel steering on
takeoff. The landing gear extension and retraction
operation took longer than normal, and the operation of the
bomb-bay doors was sluggish.
16. The shakedown mission being flown required a free-fall
operation (manual extension) of the landing gear. There was
no malfunction of the landing gear system reported. While
the right scanner was in the left wheelwell, the other
crewmember, standing by on interphone, detected gasoline
fumes coming from the right wing crawlway access port. This
fact introduced the possibility of a fuel manifold leak
near the landing gear components. In an attempt to
determine the possibility of this occurrence, an inspection
of another B-36 aircraft was accomplished. It was disclosed
that there was marginal clearance between the three-inch
fuel manifold and the main landing gear pivot shafts.
However, after review of testimony and consideration of all
known facts, there appears to be no correlation or
connection between the free-falling of the gear and the
discovery of the gasoline fumes, except that the airstream
configuration would have been changed in the wheelwell area
at the time the gear was extended, causing fumes to enter
the crawlway access port. The Aircraft Commander was
advised of the detection of gasoline fumes; however, he
evidenced no concern over this condition.
17. Except for the detection of gasoline fumes during the
manual extension of the landing gear, it appears that the
fuel system operated satisfactorily up to the time of the
emergency. Immediately prior to the emergency, fuel was
being delivered to the fuel manifold from Nos. 1, 2, 5, and
6 tanks; booster pumps were "ON." Fuel tank valves on Nos.
3 and 4 tanks were closed, with booster pumps "OFF." The
left and right auxiliary fuel tanks were shut off. No
bomb-bay fuel tank was installed. When the emergency was
declared, the flight engineer closed the No. 5 engine fuel
valve and No. 5 fuel tank valve. However, since Nos. 7 and
8 fuel manifold valves were not closed, gasoline under
pressure would be present in the fire area. If a fuel line
was severed upstream of No. 5 engine fuel valve by the
fire, explosion, or at the time No. 5 engine separated from
the aircraft, the result would be an uncontrollable
gasoline fire. Had the flight engineer closed Nos. 7 and 8
fuel manifold valves, gasoline would not be pumped into the
No. 5 nacelle area. This emergency fuel shut-off procedure
followed by the crew closely parallels the procedure as
outlined in T.O. 01-5EUC-1. The technical order should be
revised to include appropriate instructions to isolate
specific fuel tank and manifold fuel from a suspected fire
area.
18. During the inspection of a B-36 aircraft, it was
disclosed that a number of fuel system components are
closely grouped in the forward area of the Nos. 2 and 5
nacelles. There are no electrical relays or other
electrical components in this immediate area which could
produce an exposed electrical spark. This area is not
ventilated by ram air circulation; however, it is
conceivable that if a fuel leak existed in this area, fuel
in vapor form could pass from this area aft to the air
plug. If this occurred, the vaporized fuel would pass over
hot exhaust ducting and, in all probability, would be
ignited causing an immediate gasoline fire with resulting
black smoke. Considering all factors and the fact the
initial smoke was reported by witnesses to be from white to
light grey in color, it would appear that the initial fire
originated from a source other than fuel.
19. Technical Order 01-5EU-352, subject, "Sealing of Fuel
System Hose Connections - All Series RB-36 and B-36
Aircraft," had not been complied with. This technical order
was issued to eliminate a possible fire hazard resulting
from leaking of fuel line hose connections; however, it
could not be determined wether B-36D, SN 49-2661 developed
any fuel leak while in flight. The contents of this
technical order would indicate that it is a safety of
flight item and should require immediate compliance, rather
than be placed on a red diagonal status.
20. There was no indication of a malfunction of the
electrical system. Alternators Nos. 2, 3 and 4 were
paralleled on the entire bus. No. 5 alternator was excited
but in a standby status. No. 5 alternator breaker was in
the open position; therefore, no indication of its
operation was apparent to the crew. All bus-tie circuit
breakers were closed. This was the alternator configuration
for the entire flight. This procedure conflicts with the
procedures outlined in T.O. 01-5EU-355, 21 December 1951,
subject, "Operation of the Electrical System of B-36 Series
Aircraft in which Exciter Protection Relays Are Not
Installed." This technical order requires that Nos. 2, 4
and 5 alternators be operated in parallel and that
alternator No. 3 be operated on an isolated bus supplying
the loads to that bus. The technical order applicable to
B-36D, 49-2661, further states that Watt-VAR meter readings
will be observed at 15 minute intervals. This procedure was
not complied with by the flight crew. Line voltage and
frequency had been adjusted earlier in the flight by means
of a precision voltmeter during test of the K-3
Bombing-Navigation System. The voltage, wattage, and
frequency indicator readings, when read, were normal with
the exception that wattage of No. 5 alternator was always
always "ZERO" since that alternator was on standby for the
entire flight. Under the flight conditions existing before
and during the time of the emergency, there was no demand
for a large amount of electrical current. The UR and
accident history of B-36 alternators indicates that an
undetected malfunction of No. 5 alternator may have
occurred. If No. 5 alternator did fail, the crew would have
had no indication since this alternator was on standby. The
number of UR's submitted on alternators totals 115 during
the period 1 January through July 1952. During the same
period, there were 97 UR's submitted concerning the
constant speed drive. The following alternator malfunctions
could have resulted in a fire:
a. Failure of the speed governing system of the constant
speed drive would permit the alternator to rotate at
excessive speed. This could result in disintegration of the
alternator which creates excessive heat and could result in
a magnesium fire of the integral components of the
alternator.
b. Failure of an alternator bearing would cause mechanical
friction between rotor and stator with the same result as
above. With No. 5 alternator in a standby status, the above
conditions could occur without being detected from
instruments on the flight engineer's panel.
21. Two fire warning lights were found to be inoperative
during the flight: No. 2 accessory section and No. 6 power
section. However, neither of these two areas can be
associated with the cause of the accident. Fire warning
lights that were reportedly operational did not indicate
the presence of the fire in or around No. 5 engine nacelle.
A possible reason for this malfunction may be explained by
the conditions necessary to activate the Edison fire
warning system. The thermocouple unit must be subjected to
an abnormally rapid rise in temperature. If the rate of
temperature rise is not rapid enough, the thermocouple unit
will not produce sufficient voltage to energize the relay
that closes the circuit to the fire warning light. It is
believed that the rate of temperature rise of the fire that
developed in or around the No. 5 engine nacelle was not
rapid enough to activate the fire warning system. The No. 5
alternator is not in the immediate vicinity of any fire
detection units. Information obtained in the field and in
the testimony of this investigation, indicates a high
number of occurrences in which the B-36 fire detection
system failed to detect a known fire. Study of the
experience with fire detection devices in USAF aircraft
emphasizes the inadequacy of present systems to provide
reliable indications of fire.
22. The fire extinguishing system was not activated at any
time. The system is normally activated as step #5 in the
procedure for fighting an engine fire described in T.O.
01-5EUC-1. The flight engineer did not discharge the fire
extinguisher in No. 5 engine nacelle. This may have been
due to the short time interval between following the
commands of the right scanner and the Aircraft Commander's
order to bail out. The issue of detailed commands by the
right scanner instead of reporting the fire as such, was
irregular and did not follow the engine
shutdown procedure in the
operating instructions of T.O. 01-5EUC-1, neither did his
procedure follow the procedure outlined on the placard
beneath the engine selection switches of the fire
extinguishing system on the main control panel. The
Aircraft Commander was monitoring the interphone during the
emergency. However, he did not countermand any instructions
given by the right scanner, neither did he advise the
flight engineer to follow any particular procedure for
engine shutdown. All B-36D and RB-36 aircraft inspected at
Convair had the outdated placard posted in the aircraft. It
is possible that other B-36 aircraft may be placarded
likewise. The placard reads as follows:
"Emergency Operating Instructions
1. Propeller - Feather
2. Mixture Control - Idle Cut-off
3. Engine Fuel and Oil Shut-Off Valves - Close
4. Fire Ext. Eng. Sel. Switch - On (5 Secs.)
5. Heat, Anti-ice, Cooling Switches - Off
6. Pressurization Switch - On Unaffected Side
7. Discharge Switch - Alternator Position
8. Do NOT
Restart
Engine"
The procedure, as indicated above is not in agreement with
T.O. 01-5EUC-1. It is apparent, from testimony submitted,
that Convair civilian flight personnel have not been
following the procedure outlined in the technical order. It
is imperative that the fire extinguisher be activated when
smoke and/or flame is visible.
23. All radio communication equipment, including the
interphone, operated normally throughout the flight. Before
evacuating the aircraft, the radio operator using the
liaison transmitter and receiver, contacted the Convair
ground station on 3280 kcs. declaring an emergency. Other
testimony indicated that the liaison transmitter meter
readings were normal, which corroborates the flight
engineer's testimony that the electrical system was
operating at the time of the emergency.
24. The flight crew scheduled for this mission consisted of
eight personnel. They were: Aircraft Commander, copilot,
flight engineer, assistant flight engineer, radio-radar
operator (radar technician), assistant radio-radar
operator, right scanner, and left scanner. All crewmembers
were well qualified to perform their respective duties,
with the exception of the radar technician who was not
qualified to free-fall the landing gear. Only two
crewmembers were in the aft portion of the aircraft. When
the right scanner went to the wheelwell areas in order to
free-fall the landing gear, his position was unattended for
approximately 15 minutes. The left scanner remained in his
position prior to the time the emergency was declared. From
his position he did not notice any smoke passing by the
right scanner's blister; however, after the right scanner
returned to his position, the left scanner, when near the
right scanner's position, was able to observe light grey
smoke coming from the air plugs of No. 5 engine. The left
scanner explained that, due to the undercast cloud
conditions, it was very difficult to see the smoke and only
when he saw it coming from the air plug, could he follow
the smoke trail aft.
25. Although Convair standing operating procedures direct
that a life vest will be worn when flying over water, only
two crewmembers were wearing life vests when they
parachuted. Other crewmembers had left their life vests in
their A-3 (parachute) bags that were aboard the aircraft.
It appears from the testimony that there is a general
disregard for the wearing of over-water equipment.
26. There were no discrepancies noted in the aircraft
engineering forms maintained by Convair. The company
inspection forms that were reviewed contained discrepancies
covering the period 3 July 1952 to 4 August 1952. There
were a total of 23 discrepancies concerning the right wing
area. Of these discrepancies, there were two fuel leak
items and three oil leak items. Ten of the 23 items
concerned No. 5 engine nacelle and were of a varying nature
but were not significant in establishing a pattern.
According to testimony, all discrepancies were
satisfactorily corrected prior to this flight.
SUBSTANTIATING
DATA ON FILE IN DIRECTORATE OF FLIGHT SAFETY
RESEARCH
27. The following data pertaining to aircraft accident
investigation of B-36D, SN 49-2661 are on file in the
Directorate of Flight Safety Research and can be obtained
on request:
A. Special Orders Directing the Investigation
B. Western Air Procurement District Investigation Orders
C. Statistics
D. Statements of Witnesses
E. Testimony of Flight Crew to Convair
F. Testimony given to Aircraft Accident Investigation Board
G. Sequence of Events
H. CAA Incident Report (Tower Report)
I. Flight Clearance Form
J. Aircraft Flight Release Form (Form F Data)
K. Discrepancies Written up on B-36D, SN 49-2661
L. Check Sheets, B-36D, SN 49-2661
M. Convair Flight Safety Manual and B-36 Flight Crew
Indoctrination
N. Map of Crash Area
O. Statement of Examination of Recovered Wreckage
P. B-36 Aircraft Accident History
Q. Photographs
RICHARD J. O'KEEFE
Brigadier General, USAF
Director, Flight Safety Research