Crystal Mountain DC-6 Accident Report

The following are Ron's hand-typed excerpts from the CAA accident report investigation. Some abbreviation and omissions  and lots of typing mistakes are made.

Adopted: December 7, 1951                     Released: December 12, 1951

UNITED AIRLINES INC. ,  - FORT COLLINS, COLORADO,  JUNE 30, 1951

THE ACCIDENT

  United Air Lines' Flight 610, a Douglas DC-6, N-37543, crashed 18 miles west-southwest of Fort Collins, Colorado, and 29 miles west of the centerline of Airway Amber 3, at approximately 0200, June 30, 1951 All of the occupants were killed and the aircraft was demolished.

HISTORY OF THE FLIGHT

   Flight 610 of June 29, 1951, originated in San Francisco, Ca. and was scheduled to Chicago,Ill., with an en route stops, among which were Oakland, Ca., Salt Lake City, UT; and Denver, CO. The crew consisted of Capt. J.R. Appleby, First Officer H.G. Tower, Flight Engineer A.T.Petrovitch, and Stewardesses C.J. Raymond and F.m. Smith. The flight departed San Fran. on schedule at 1915 and after stopping at Oakland proceeded to Salt Lake City, arriving there at 2324. It departed Salt Lake City at 0011, June 30, 1951, 26 minutes behind schedule due to the reloading of bulky cargo. At the time of departure the aircraft weighed 78,597 pounds, which was within the certificated gross take-off weight of 79,380 pounds; the load was properly distributed with respect to the  center of gravity. There were five crew members, forty-four adult passengers and one infant on board. The approved flight clearance indicated an IFR flight, via Red Airway 49, Green Airway 3, and Amber Airway 3 , to Denver at a cruising altitude of 15,000', with Omaha, Neb. designated as the alternate airport.

   The flight proceeded in a routine manner and at 0104 reported over Rock Springs, Wyo. at 15,000', estimating its arrival over Cheyenne, Wyo. at 0147 and over Denver at 0207 (All radio communications from the flight while en route from SLC to Den were transmitted to the company radio operator at Den and relyayed to Denver Air Route Traffic Control)  Forty-three minutes later, at 0147, the flight reported having passed the Silver Crown fan marker (located 12 miles west of Cheyenne) and requested a lower altitude. Accordingly, a new clearance was immediately issued- "ARTC clears United 610 to DuPont intersection,(The intersection of the west course of the Denver VAR range with the north course of the Denver low frequency range) descend to 8500' immediately after passing Cheyenne, maintain 8500 feet, no delay expected, contact approach control over Dacono (Dacono is a fan marker located appx 15 miles north of the DuPont intersection)"  This clearance was acknowledged and the flight reported that it was over Cheyenne at 0147, at 15,000' and was now starting to descend.  The Denver altimeter setting was then given the flight as being 30.19 inches. Nine minutes later, at 0156, the flight reported reaching its assigned altitude of 8500'. No further communication was received from the flight.

  At 0200, the Denver Control Tower requested the company radio operator to advise the flight to call approach control. Repeated calls were made without an answer. It was later determined that Flight 610 had crashed on a mountain 18 miles west-southwest of Fort Collins, CO.

INVESTIGATION

   Investigation disclosed from the direction of the swatch cut through the trees that the aircrafs struck the side of Crystal Mountain while flying with its left wing low and on an appx magnetic heading of 210 degrees. The altitude at the point of impact was found to be 8540' MSL. After initial contact with the trees the aircraft continued to travel appx sixty feet, at which point it struck the ground. From here it traveled in a straight line 225 feet, then bounced into the air again, and came to rest 465 feet farther on. The aircraft was demolished, and aircraft parts and assemblies were strewn over a 1400' area. Localized fires occurred after impact.

  An examination of the wreckage revealed that at the time of impact the landing gear and flaps were retracted. During this examination nothing was found to indicate that there was any structural failure of the aircraft or its components prior to impact. Numerous pieces of mail, paper, cabin insulation and other light materialas were found NNE of the point of  impact along the flight path a distance of two miles from the scene of the accident. All of the debris was heavily spotted with engine oil and several pieces showed evidence of burning, indicating that at the time of impact an explosion occurred which blew this material aloft and that it was carried away by the eddying wind currents. Identical material which was also spotted with engine oil was found at the scene of the accident.

  The damaged engines and propellers were examined and these indicated that all four engines were developing considerable power when the impact occurred. All engine instruments were so severely damaged that their readings were of no value.

   A study of the aircraft's maintenance records indicated the aircraft was airworthy when departing San Fran.

   Much of the radio navigational equipment and some of the flight instruments were recovered. These were taken to Denver for study and analysis. The resulting investigation indicated that prior to the crash no fire existed in any of the electronic or electrical equipment, and that all of the aircraft's comm. and nav. equipment was apparently functioning ina normal manner. Conditions of propagation dfuring the times involved were conducive of good radio reception. All ground radio stations in the area were functioning normally, as evidenced by subsequent flight checks and a study of each station's records. The aircraft was heading 210 degrees magnetic, plus or minus a few degrees, at the time of impact.

  This last fact is further substantiated by the condition of the directional instruments when recovered. In the cockpit were four heading indication instruments. There were two magnetic or master direction indicators operated by a flux gate compass system, one each for the pilot and copilot. These were both jammed at a reading of about 210 degrees. The magnetic compass and the DG were also found to be reading appx 210 degrees. Furthermore, as a part of the radio nav. equipment there were two ADF receivers. The dual indicator azimuth scale of the copilot's ADF must be rotated manually and when used to determine a bearing it is set to agree with the magnetic heading of the aircraft. This instrument was found jammed at a reading of appx 202 degrees.

   On each side of the control pedestal of the DC-6 are panels containing six audio selector toggle switches. The two switches nearest the captain actuate the voice and range control positions of that pilot's ADF, the two middle switches actuate the same controls on the VHF nav. receiver, and the two furthermost form the captain actuate  identical controls on the copilot's ADF. These switches are in such a position that they cannot be easily seen by either pilot and to use them at night without the use of lights it is customary to feel for them. All switches are of uniform size and are equally spaced on the panel. Although cockpit lights and a small flashlight are available to the captain, it is normal practice to use a minimum of cockpit lighting to avoid glare.

   The magnetic course to Denver from Cheyenne is 168 degrees. The audio signals of the Denver low freq. range for this course are heard as an "A" on the left side and an "N" on the right side.  At Denver there is another range, namely a VAR (Visual Aural Range), the norch course of which nearly parallels the north course of the low freq. range.  The audio signals of this course when flying toward Denver are heard as an "N" on the left or east side, and as an "A" on the right or west side. The similarity of the tone of the signals emitted by both ranges makes it difficult to differentiate between them. The ID signal "DEN" is the same for both stations. 

   As a part of the investigation, an exploratory flight was made in a similar type aircraft to determine the credibility of the probable flight path of the subject aircraft between Cheyenne an the scene of the crash.  In an effort to duplicate the assumed track of Flight 610, the test flight crossed the Cheyenne range station from the NW at 15000', and a shallow descending right turn was started toward a heading of 210 degrees magnetic. Two minutes were required to arrive at this heading. Continuing on this heading, a descent of 700 to 1000 fpm was maintained at an indicated air speed of 245 mph. Descent from 15000' to 8500' MSL required seven minutens. Four minutes later the flight arrived over the scene of the accident after climbing slightly to clear the ridge. Thirteen minutes were required to fly from Cheyenne to the scene of the accident. This time, addeded to the time the aircraft reported crossing Cheyenne, closely approximates the assumetd time of the crash.

   Recorded radio contacts with Flight 610 disclosed that between Salt Lake City and Cheyenne it was flown in accordance with the flight clearance.

   Captain Appleby had been employed by UAL, INC. since Nov. 1, 1940 and had accumulated a  total of 9,900 flying hours in DC-3 equipment. Prior to April 1951 he had flown many hours as captain in DC-3 aircraft between SLC and DEN.  In April he attended the compny's DC-6 school, where he received 15:50 horus of transition training. Following the DC-6 training he received 9:20 hours of training on DC-4 type aircraft. This latter training was necessary, since Capt. Appleby was being assigned as captain on a route designated as San Fran- East, involving both DC-6 and DC-4 aircraft. Up to and including June 30, 1951, he had flown 29 hours as first officer and 61 horus as captain of DC-6 aircraft.  The records also indicated that he had made 11 one-way trips in and out of Denver as captain in this type aircraft.

  First Officer Tower, who was assigned as Captain Appleby's copilot, had accumulated 5,848 flying hours, of which 1,526 were on DC-6 aircraft and 917 on DC-4's. Both pilots were well acquainted with the terrain which lies to the right of the route between Cheyenne and Denver.

   On the night fo June 29-30, a weak upslope flow of air existed on the east slope of the Rocky Mountains is southeastern Wyo. and northeastern CO.  This resulted in cloud layers ranging generally from 8,000' to 17,000'.  Previously, a general shower and thundershower condition existed in the area but by the night of June 29-30 only scattered light showers remained in SE Wyo. and no thundershowers existed nearer than the eastern border of CO.  There was a solid layer of clouds south of Cheyenne with base 8000' and top 12000'.  No turbulence or icing of significance was indicated for that area. For this area winds aloft between 8000' and 10,000' were northerly and under 10 mph. This was substantially as forecast.

ANALYSIS

   Numerous theories were explored in an effort to determine why the pilot, after crossing Cheyenne, possibly assumed a heading of 210 degrees magnetic and then held this heading until the aircraft crashed into the mountain.  One plausible theory is that after the aircraft passed over the Cheyenne range station the Denver LF range was tuned for aural directional guidance to Denver. At the same time the Denver VAR range was tuned in for the purpose of identifying the DuPont intersection, the point to which the flight was cleared. This intersection is the point where the west course of the Denver VAR range crosses the north course of the Denver LF range.

   In order to isolate the LF range receiver to aid in its aural reception, the captain may have meant to eliminate the aural signal of the VAR range receiver by depresing the toggle switches (voice and range) which are mounted on the audio selector control panel located near the captain's right knee. As previously stated, in a darkened cockpit the lights must be turned up in order to see these switches and read their positions; however, instead of doing this it is often the practice to feel for them.

  It is therefore possible that the captain may have inadvertently depressed the wrong switches, the second and third switches from the left, thinking he had depressed the third and fourth (or middle two) switches. This would  silence the range signals of the captain's LF receiver and also silence the voice feature of the VAR receiver, but would permit the VAR range signals to be audible. As previously stated, the ID signals and tonal qualities of both ranges are identical.

   After the aircraft passed over the Cheyenne range station, the normal procedure would be the execution of a standard rate right nurn to a heading, probaly not exceeding 210 degrees, which would intercept the north course of the Denver LF range. With the above-mentioned configuration of radio tuning, the "A" signal is on the left (east) side of the north course of the Denver LF range. Also the signal "A" is on the right)west) side of the north course of the Denver VAR range.

  It can be seen that on approache to Denver from th enorth, a right turn to attempt to fly the "on course" of the LF range while listening to the "A" (right) side of the VAR range would take the aircraft deeper into the "A" quadrant of the VAR range and thus an "on course" signal would never be heard.

   As previously noted, the records indicate that Capt Appleby had flown the route involved many times in DC-3 aircraft and that he had made eleven one-way flights to and form Denver as captain in DC-6's.  Since the DC-6 aircraft is much faster than the DC-3 the difference in air speed may not have been properly considered resulting in the heading toward the mountains being maintained longer than should have been.  However, no logical explanation can be found for the length of time the aircraft was held on a heading which the crew shouldl have known would lead to the mountains west of the airway.

  Another possible theory was considered which was subsequently established by a flight test conducted by the CAA.  After passing Cheyenne, the CAA pilot tuned his ADF to the Denver LF range and turned that receiver's selector switch to the compass position. In tuning the Denver frequency of 379 kilocycles he purposely detuned the receiver on the high side. This detuning allowed the receiver to be affected by the range signal of Fort Bridger, Wyo. (located approximately  304 miles WNW of Denver), the frequency of which is 382 kilocycles. As a result the ADF compass needle indicated an average bearing of 225 degrees on the azimuth scale but with the needle "hunting" plus and minus 20 degrees.   With the ADF switch in the compass position and with fine tuning it wa possible to receive a faint "A" signal and a "DEN" identification.  However, it should be noted that when the Denver LF range was properly tuned the signals were clear and distinct.  Therefore, if the UAL captain had inadvertently detuned his ADF as described above, and was following such a heading thinking the needle indicated the direction of the Denver range station, he would have been flying toward the mountains.

   The above-mentioned theories are based on the  premise that the pilot tuned to the  Denver ranges after passing Cheyenne.  However, the Cheyenne LF range provides an excellent airway course to the south, meeting the north coruse of the denver LF range.  Had the Cheyenne LF audio facility been utilized to a point approximately halfway to Denver and  had the Denver range then been properly tuned, no difficulty would have been experienced in receiving correct ADF indications and clear aural range signals.

   The Denver VAR range was commissioned Jan 1, 1946. This facility was installed with the approval of the aviation industry, as is the case in andy addition or major change in all radio aids to air navigation.  although for five years this range has operated in close proximity  to the Denver LF range and although both ranges utilize the sam e "DEN" ID, there have been no known recorded complaints form airmen that difficulty or confusion resulted.

   However, in the interest of safety and in order to avoid any possible error in id'ing these ranges, the CAA has placed the code letter "V" before the "DEN" id signal of the VAR range. This additional signal should avoid any possible mistake in confusing the tow ranges.

   Also, as a result of this accident, UAL has effected a change in the audio selector panels which contain the siz selector switches on all their DC-6 aircraft. This was accomplished by lengthening the middle two toggle switches which select the VAR ond other VHF radio nav receivers, and was done to help avoid any possible mistake  by the crew in switch selection.

   Subsequent to the investigation and public hearing relative to this accident, the CAB was infgormed by UAL that it has reviewed its entire flight ops admin. This review indicated, among other things, that greater importance should be placed upon indoctrination and training of flight personnel, with particular emphasis on maintenance of route and equipment qualification. It is understood that the program is at this time in the process of development and that UAL will make it a continuous effort.

FINDINGS

  On the basis of all available evidence the Boad finds that:

1. THe carrier, the aircraft, and the crew were properly certificated.

2. The gross weight of the aircraft was within approved limits, and the load was properly distributed with respect to the center of gravity.

3. There was no malfunctioning of the aircraft or any of its components prior to impact.

4. Although instrument flight conditions existed at the time of the accident, no abnormal weather was encountered by the  flight.

5. The flight crossed the Cheyenne range station at 15,000' and then executed a right turn to a heading of 210 degrees magnetic, descending to 8,500'.

6.  The 210 degree heading was maintained until the aircraft struck the mountain.

PROBABLE CAUSE

  The Board determines that the probable cause of this accident was that, after passing Cheyenne, the flight for reasons undetermined failed to follow the prescribed route to Denver and continued beyond the boundary of the airway on a course which resulted in the aircraft striking mountainous terrain.

BY THE CIVIL AERONAUTIS BOARD: (signed) Donald W. NYROP, Oswald RYAN, Josh LEE, Joseph P. ADAMS, Chan GURNEY

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Last Modified: 1/1/2008