.,SA-I 53 File No. 1-0097-47 ) CIVIL AERONAUTICS BOARD

I ACCIDENT I W ESTIGATOO I

I Adopted: February 2, 1948 . Released: February 3, 19y8

UNITED AIR LlhES, INC., BRYCE CANYON, , OCTOXR 24, 1947 The Accident - United Air Lines I Flight 608 crashed observed what appeared 'to be white smoke at 1229 MST,2 October 24, 1947, 1 1/2 streaming from the aircraft, followed miles southeast of Bryce Canyon , later by dense black smoke. The first Utah, during an attempted emergency witnesses who observed fire in the bottom landing resulting from a fire in flight. of the aircr8ft at approximately the The aircraft, a Model DC-6, NC 37510, center-section were located approxi- was demolished by impact and fire, and mately 15 miles south of Bryce Canyon. all of the 46 passengers and the crew of Until shortly before the moment of im- 6 were killed. pact,. the aircraft appeared to be under normal control; hawever, no_,witnesses History of the FI ight were located who observed the, crash. Flight 608 departed , Cal- ifornia, at 1023 with its destination I nvest i gat ion , Illinois, to cruise at 19,000 Immediately after the accident the feet according to visual flight rules. wreckage was protected by Civil Aeronau- Routine position reports were made over tics Administration personnel until a Fontana, Daggett and Silver Lake, Cali- guard was established by the National' fornia; Las Vegas, Nevada; and Sdint Park Service, pending the arrival of George, Utah. During the latter report, Board investigators. During the evening the flight indicated that it estimated of the same day various investigation passing over Bryce Canyon, Utah, at 1222. groups were organized and an inspection At 1221 Flight 608 reported that a of the wreckage and a search of the flight fire had been detected in the bagggge path were begun. Parts of the wrecked compartment which the crew was unable to aircraft were transported to the Douglas extinguish. The report added that the Aircraft Company plant at Santa Monica, cabin was filled with smoke and that the California, as rapidly as possible in flight was attempting to make an emer- order that identification and evaluation gency landing at Bryce Canyon Airport. as well as reconstruction of the perti- Shortly thereafter the flight again re- nent structure, might be facilitated. ported that the nt&ilis going out-we The latter reconstruction included the may get down and we may not." At 1226 major portion of the fuselage from the an6ther transmission was received from leading edge of the wings to the rear the flight indicating that it was going pressure bulkhead. A separate mock-up Into the "best placen available. One of the air-conditioning compartment was minute later the flight reported wwe may also made as a means of tracing the flame make it-approaching a strip." No fur- path and assessing the damage resulting ther contact was had from the flight. from fire in flight. Witnesses who observed the aircraft The aircraft struck the ground at a as it was approaching Bryce Canyon from point approximately 1 1/2 miles southeast approximately 20 miles southwest first of the Bryce Canyon Airport while headed in a westerly direction. The flight path 'The Board is making further analysis of the con- was projected from the point of impact slderable amount of technlcal data complled in the southeastward in a long gentle right course or its lnrdstlgatlon of thls accident and sfm- llar accldent at Gallup, New Mexico and a report wlll curve which eventually swung southwest- be made at a later date containing a complete analy- ward toward Tropic, Utah. Various arti- sts of all pertlnent data and further discussion or the several factors contrlbutlng to the cause of this cles carried aboard the aircraft and acc dent. kAll tlmes referred to herein are Mountaln Stand- component parts of the aircraft structure ard and baaed on the eehour clock. were located along the flight path for a -18416 (1)' ... ' 2 Accident Invest i gat i on Report

maximum distance of 26 miles from the Examination crf the powerplants, hy- point of impact. The sequence of struc- draulic system, electrical system and tural failure resulting from the fire cabin supercharging system indicated that aboard the aircraft was readily recon- none of these sFstems contributed to the structed from these objects. It was ap- fire in this imstance. Inspection of the parent that the aircraft structure began primary wing an$ empennage structure and to disintegrate in the center section in the control surfaces gave no evidence of the vicinity of the right wing fillet failure in flight. and that parts of the interior of the At the time af departure from Los aircraft in the proximity of this area Angeles all four main and the four al- began to fall from the aircraft early' in ternate tanks were filled to capacity; the development of the fire. The exten- both auxiliary tanks were emyty. Be- siveness of this disintegration is in- cause of illness, the captain's regular dicated by the fact that the trailing co-pilot did not accompany him on this edge of the right wing flap, the main flight. The testimony of the captain's cabin entrance door and the buffet cold regular co-pilot indlcates the procedure box had fallen from the aircraft prior which the captafn normally followed with to impact. The extent of burning on respect to fuel management: The take- parts of the cabin interior, prior toim- off and climb would be accomplished with pact, indicates conclusively that the each engine drawing fuel from its re- fire in flight was of such severity as spective main tmk. Immediately after to have been unsurvivable for cabin reaching cruising altitude the captain occupants. would switch each engine to its alter- Reconstruction of the fuselage and nate tank. In this manner the flight analysis of the burning of its struc- would proceed until a minimurb of 500 tural components indicate that the burn- pounds of fuel remained in the lowest

ing in flight took place in an area cov- alternate tank. At this poifit, the cap- ~ ering the lower right side of the fuse- tain would transfer fuel from the higher lage beginning at a point in the center of the alternate tanks to the lower in section approximately midwing and ex- order to equalize the contents of all tending rearward approximately 23 feet alternate tanks. This procedure requires and upward along the right side of the placing the left cross-feed in the Ron fuselage to the top of the window line. engines 1 and 2" position and the right Inspection of the structure and compo- cross-feed in the "on engines 3 and 4" nents of the cockpit and the rear lounge position; the booster pump switches for and toilets show no evidence of burning the respective out-board alternate tanks in flight. in the *highn position. After having Control cables passing through the equalized-the contents of the alternate air-conditioning compartment, commonly tanks, the captain would stop the trans- referred to as the nboiler room," were fer process by turning the booster pump found to have been partially consumed by switches and the cross-feed controls to fire and it was evident that all of these the noff" position. cables had failed in tension in the The Model DC-6 as delivered by the burned area. At least one of the emer- manufacturer was equipped with. a fuse- gency landing flares which are located lage fire extingulshing and detecting at- the trailing edge of the right wing system which included protection for both fillet immediately forward of the alcohol forward and aft baggage compartments and tank was found to have been ignited in the hydraulic accessories compartment. flight. Chemical analysis of smudge However, with the exception of a 1.4 marks on aircraft parts and components pound bottle of carbon dioxide which indicates clearly that the damage result- lischarged 'directly into the cabin heater ing from the burning of the barium ni- 2ombustlon cnamber, no fire-extinguish- . trate in the flares covered an extensive ing protection was provided the air-con- area aft and above the flare location. iitioning accessories compartment. The alcohol tank which had also fallen Inspection of the DC-6 fuel system from the aircraft prior to impact showed lisclosed that the No. 3 alternate tank p signs of severe external burning and in- vent outlet w'as located on the right side dicated the probability of having been of the fuselage near the leading edge of ruptured before it left the aircraft. the wing and close to the bottom wing -18416 .. Accident lnvesti gation Report 3

fillet. Approximately 10 feet aft of during flright. NO Instructions had been this point and slightly to the left there given the sir carrier's pilots concerning was an air scoop which served as a source possible hazards associated with over- -of cabin heater combustion air and coql- flow of gasoline from the No. 3 alter- ing air for the cabin supercharger air nate tank. No instructions were pro- after-cooler and cabin supercharger oil vided in the manufacturer's DC-6 Opera- cooler. Flight tests conducted with tioq Manual, or the CUApproved DC-6 other model DC-6 aircraft subsequent to Aircraft Operating Manual advising this accident revealed that overflow from against fuel transfer, nor were any in- the No. 3 alternate tank through the air structions contained in the air carrier's vent line and out the vent outlet would DC-6 Pilot's Operating Manual outlining sweep back in the slip stream toward the any procedures for fuel transfer. cabin heater combustion air intake scoop and that a considerable quantity of fuel Discussion would enter the scoop. Ground tests Investigation of this accident leaves clearly demonstrated that, under con- little doubt that the initial fire and ditions simulating the entry of fuel the most extensive burning which followed overflow into the scoop in flight while were caused by the combustion of aviation the heater was opera$ing, the cabin fuel.' Reconstruction of the flame path heater could be expected to backfire and indicates the origin of this fuel to be thereby propagate flame downstream into forward of and approximately in line the air scoop. Incoming fuel would, with the cabin heater combustion air in- thereafter, be expected to continue to take scoop. Since the only source of burn in the air scoop and duct. gasoline in this area is the No. 3 al- * Chemical analyses of smudge stains ternate tank vent outlet, it is con- were made of an extensive area of the cluded that gasoline overflow through aircraft and these analyses indicated this outlet enfered the scoop while.in that the burning of the aircraft struc- flight, was ignited in the heater and ture in flight was primarily the result thereafter burned in the scoop and duct. of the combustion of leaking or over- In view of the above, the testimony flowing fuel. This burning was epgravated of the captain's regular co-pilot con- in a more localized area by the burning cerning the flight procedures routinely of the barium nitrate of a flare. These .followed by the captain indicates very analyses when viewed in the light of the strongly that, in the course of trans- sequence of the parts which fell from ferring fuel from the outboard alternate the aircraft in flight revealed that the tanlcs to the inboard alternate tanks in earlier burning was the result of gaso- flight, he failed to stop the transfer ,line fire. Of the parts which fell from In time to prevent the No. 3 alternate the aircraft, those bearing signs of tank from overflowing through the vent barium nitrate burning were first located outlet. The time of discovery of fire along the flight path at poinis which aboard the'alrcraft corresponds very corresponded to the area in which visible closely to the time at which the crew fire was first observed. would be expected to transfer fuel from According tg the testimony of the the outboard alternate tanks to the in- manufacturer's representatives, the DC-6 board alternate tanks. fuel system was not designed for fuel Early in the course of the Investi- transfer between tanks. However, it is gation it became apparent that the emer- apparent that this system is readily gency landing flares contributed to the adaptable to fuel transfer and was, in severity of the fire in flight. Although fact, extensively employed for this pur- it was apparent that a fire of consider- pose prior to the aqcident. Testimony able intensity would have been necessary of representatives of Douglas Aircraft to ignite these flares, the hazard' that Company, the Civil Aeronautics Adminis- their location in the Model DC-6 pre- tration, and air carriers operating DC-6 sented was, nevertheless, recognized. aircraft disclosed that no tests were As a result of this investigation, the conducted prior to certification of this Board promulgated a special regulation model ai'rcraft to determine whether any - enabling air carriers operating this hazard existed through possible overflow model aircraft to remove all landing of fuel from the vent outlet into the flares until proper looation In the sir- cabin heater combustion air Intake scoop craft is made. -18415 Accident Investigation Report

Soon after the investigation was ini- system. Similarly, other pertinent mod- tiated, a Modification Board was organ- ifications will be necessary before the ized, consisting of representatives of aircraft is periltted to be operated with the Civil Aeronautics Board, the Civil the cabin supercharging and thermal de- Aeronautics Adminis tration, Douglas Air- icing system in use. craft Co., Inc., United Air Lines, Inc., Because of the extensiveness of the and , Inc., the purpose Modification Board's analyses of the of which was to analyze and improve where DC-6, several conditions were disclosed possible the component parts and systems which, while not directly related to of the DC-6 in the light of technical this accident, nevertheless are suscep- data compiled by the manufacturer and tible of material improvement. These the operators, as well as the findings items were also included in the Board's of this investigation. This Board com- recommendations to the Civil Aeronautics pleted its studies early in December and Adrninistration(s Type Certification Board submitted a list of proposed modifi- and the modification proposed in connec- . cations to the Civil Aeronautics Admin- tion therewith will no doubt serve to istration's Type Certification Board. improve the efficiency and the over-all This list was approved as submitted, safety of this model aircraft. although the latter Board suggested ad- The investigation clearly es tablished ditional items to be included. These that the origin of the fire in this in- item were added to the modification stance was not in either of the baggage list, the total of which constitutes the compartments. The Board has been cogni- basis for the alterations of the Model zant of the public concern over the pos- DC-6 which are presently being accom- sibility of a fire being started in plis hed . flight as a result of discharge of photo Since the industry voluntarily with- flash bulbs carried in passenger baggage drew the DC-6 from scheduled servicg and this subject was given,careful study November 11, 1947, a portion of this list during the investigation. As a result . constituted the minimum modificationplan of tests conducted by the Air Forces and to be completed before this model is re- by General Electric Corporation it has entered in service. This modification become apparent that the inadvertent plan requires the relocation of the Nos. discharge in flight of such bulbs in an 2 and 3 alternate tank vent outlets to aircraft interior is highly improbable areas in which no hazardous fuel over- and that, if discharged by any means, flow conditions will exist. Guards are the amount and rate of heat dissipated required for all fuel booster pump is insufficient to ignite even the most switches. In addition, extensive modifi- highly inflammable materials normally cation to the electrical system is re- carried in baggage or express. quired to increase the protection agairst possible fire hazards from this source. Probab Ie Cause Other modifications encompassing the power plant and fire extinguishers, as The Board determines that theprobable well as provision for drainage and added cause of this accident was the combustion precaution against fuel leakage, are of gasoline which had entered the cabin -being effected. It appears, therefore, heater air intake scoop from the No. 3 that the manufacturer and the operators alternate tank vent due to inadvertent are taking adequate action to assure the overflow during the transfer of fuel from elimination of that design deficiency the No. 4 alternate tank. Contributing which caused the fire in this instance. factors were the improper location of After the DC-6 has been returned to the No. 3 alternate tank air vent outlet service, according to the above plan, it and the 'lack of instructions provided will be limited to certain categories of DC-6 flight crews concerning hazards as- operation until the remainder of the sociated with fuel transfer. modification list is completed. For in- (~tance,operation with the heater will BY TRE CIVIL AERONAUTICS BOARD: not be permitted until extensive modifi- cations have been completed of the cabin supercharging and ventilating system, Is/ OSUALD RYAN cabin heating system, thermal de-icing fsf EARLLEB BRANCH -..-La- --> fire detection and suppression ./SI JOSH LEE ~IJ3416