DC-6 crash near Mount Carmel

Douglas DC-6 model 477-B with registration number NC37506 (there is also an incorrect version - N37506, factory - 42871, serial - 12 ^[1] ) and was released in March 1947 with the name Mainliner Utah , and was received by the American customer on March 25 United Air Lines Airlines. The total operating time of the liner was 1245 hours, including 550 hours from the last overhaul ^{[* 1]} , 87 hours from the last inspection in form No. 3 and 26 hours from the last inspection in form No. 2 . The four piston aircraft engines were the Pratt & Whitney R-2800 and equipped with Hamilton Standard propellers , model 43D60-3. From the moment of the last overhaul, engines No. 1 , 2, and 3 had an operating time of 550 hours, engine No. 4 - 187 hours. The operating time of propeller No. 1 was 320 hours, for screw No. 2 it was 118 hours, and for screws No. 3 and 4 it was 467 hours ^{[2] [3]} .

In the cockpit there was a flight crew consisting of two pilots ^{[2] [4]} :

• The aircraft commander , 35-year-old George Warner, Jr. ( born George Warner, Jr. ) received DC-6 pilot qualification on May 28, 1948. He had a total flight experience of 7310 hours, including about 30 hours on DC-6 aircraft.
• The co - pilot - 26-year-old Richard C. Schember ( born Richard C. Schember ), received the DC-6 pilot qualification on June 12, 1947, and completed additional courses on March 14, 1948, as the plane had changed by then. He had a total flight experience of 3289 hours, including about 129 hours on DC-6 aircraft.

It can be noted that the second pilot had more experience working on the DC-6 than the commander, which was caused by the policy that existed at that time in the airline when more experienced co-pilots were put in the crew for the commanders recently assigned to a new type of aircraft ^{[4 ]} .

Two stewardesses worked in the cabin ^[2] :

• 28-year-old Lorena R. Berg
• 24-year-old Nancy L. Brown

On that day, the plane operated a UA-624 transcontinental flight en route San Diego - Los Angeles - Chicago - New York . The first two stages went fine and at 09:52 ^{[* 2] the} airliner landed in Chicago. Here, the aircraft underwent a routine inspection and, if necessary, repairs, as well as a change of crew. A total of 39 passengers, 4 crew members, 2568 pounds (1165 kg) of cargo and 1800 gallons of fuel were on board, while the weight and alignment did not go beyond the established limits. At 10:44, flight 624 flew out of Chicago and headed for New York, rising to a planned altitude of 17,000 feet (5,200 m ) ^[5] .

At 11:55, the ship's commander contacted La Guardia's New York airport and reported that everything was okay with the plane and that it could fly back. At 12:25 a.m. was reported of the passage of Philipsburg that in the state of Pennsylvania and at a distance of 500 miles (800 km ) east of Chicago, and at 12:27 the crew gave the usual confirmation that it had accepted permission to reduce to a height of 13,000 feet (4000 m ) to 11,000 feet (3400 m ) ^[5] .

After only four minutes, at 12:31, the radio operator at La Guardia Airport heard someone calling themselves loudly and urgently without calling themselves. This broadcast was also heard by the crew of the Douglas DC-3 aircraft, which, like the NC37506, belonged to United Airlines, but followed behind flight 624 and a little lower. His crew dismantled New York calls in this loud broadcast. Then the stranger, after a slurred transmission, said: We are carrying out an emergency reduction . After analyzing the air situation, it became clear that this broadcast was from flight 624 ^[5] .

According to eyewitnesses on the ground, at a distance of 31 miles (50 km ) northwest of the scene, they saw a large four-engine plane, which was heading southeast to Shamokin . Maintaining a southeastern direction, the airliner flew over the Sunbury airport at an altitude of 4000 feet (1200 m ), after which it made a small left turn north of Shamokin and at an altitude of only 500-1000 feet (150-300 m) from the ground, side of the elevated area, where the hills were 900 feet (270 m ) high , and after 5 miles (8.0 km ) - about 1,600 feet (490 m ) and continued to rise east, although there was a suitable area for emergency landing on the side. Having flown over Shamokin just 200 feet (61 m ) from the ground, Douglas began to make a right turn with a climb. Then, when flying north of Mount Carmel, the roll began to increase rapidly, after which, at an altitude of 1649 feet (503 m ) above sea level and at a distance of 3 miles (4.8 km ) east-northeast of the city, flight 624 crashed into the earth ^[5] .

The airliner fell on the territory of an electrical substation , carrying a 66,000 volt transformer and breaking high-voltage wires. The ensuing explosion destroyed the aircraft, scattering its debris over an area measuring 580 feet (180 m ) by 175 feet (53 m ), and also killed all 43 people on board. The shutdown of the substation was recorded at the steam power plant in Culpmonte ^{[* 3]} , thereby recording the time of the disaster - 12:41 ^[5] .

Aircraft Technical Condition

Checking the entries in the aircraft technical journal did not find any serious comments on it, and at the time of departure from the last airports the plane was technically sound. In addition, less than an hour before the crash, the crew contacted New York and reported that the airliner was in good condition and suitable for a return flight. The engine check did not show any signs of failure during the flight, and the nature of the propeller damage indicated that all four engines were operating normally when they hit the ground. The chassis at the time of the disaster was raised and fixed; the position of the flaps could not be determined due to the strong destruction ^[6] .

Checking Fire Systems

In their last radio message, the crew reported an emergency reduction, which could indicate a fire on board ^[6] . It is worth noting that the disaster on board the NC37506 was the second in the history of the DC-6, and the first occurred 8 months earlier in Utah , and that also belonged to United. The cause of the disaster was a fire caused by the imperfection of the fuel pumping system and ventilation of the fuel tanks, when excess fuel flowed out through the ventilation holes, and then reached the interior heating system, where it ignited. After the accident, the airlines equipped the aircraft with fire detectors, and installed carbon dioxide fire extinguishers.

When studying the debris of the NC37506, six 15-pound (6.8 kg) bottles of CO ₂ were found in the front cargo bay. These bottles were empty, and their valves were open, and, as the examination showed, they were opened even before they hit the ground, that is, the crew did this intentionally, trying to put out the fire. Then all the fire alarm sensors located in this compartment were checked, but none of them could be found traces of soot, smoke or exposure to fire. The smoke detector test also did not detect smoke traces in it. And indeed, no traces of the fire were found on any of the wreckage of the aircraft before the collision with the ground, that is, there was really no fire on board ^[6] .

To understand why the crew decided that there was a fire on board, it is worth paying attention that at that time there were quite frequent cases of fire alarms. Including only in the first half of the year (from January 1 to June 30) 1948 on civilian planes there were 22 false alarms of fire sensors in the fuselage compartments and 285 false alarms of smoke sensors. Such frequent false alarms even led to unscheduled landings, so on April 28, 1948, the Civil Aeronautics Administration sent instructions to all aircraft operators to turn off smoke detectors, since false alarms were caused by violations in the device itself, and not due to improper installation or operation. From January 1 to early May 1948, there were already 44 false fire warnings on United planes, but they did not turn off the smoke detectors, believing that they could soon be replaced with more reliable ones ^[6] .

When checking the wreckage of the cockpit, a manual control panel was found for cockpit pressure control valves in the cockpit, which is located to the right of the second pilot's seat and with the help of regulators on it you can change the air pressure in the cockpit, and at the same time ventilation. When investigators examined this panel, they found that the knobs of the regulators were in the position that corresponds to the closed valves. The boost compressors themselves are located at the external power plants ( No. 1 and 4), and their examination showed that they did not rotate at the time of the disaster, that is, they were turned off. Why the crew turned them off during the flight could not be determined. Meanwhile, pressurization of the aircraft is necessary, because the DC-6 fuselage is actually not quite tight. In addition, the compartments on the plane are also communicated, and therefore air from the cargo compartment can enter the cabin and the cabin, that is, smoke, or even carbon dioxide, can also penetrate if carbon dioxide fire extinguishers are activated. So that in the latter case, the concentration of carbon dioxide in the cabin does not reach a dangerous value, there was the following procedure in case of fire ^{[6] [7]} :

1. Cabin pressurization in "Off".
2. Emergency pressure control - fully open. If the bypass valve is opened first, the alarm will sound, as there is a danger that carbon dioxide will fill the cabin and the cabin.
3. The CO ₂ queue switch is fully on.
4. 15 seconds after selecting a queue, completely empty it.
5. Start lowering immediately to a safe height.
6. Before using the second stage, first completely switch the switch to it, and then empty the queue completely.
7. If the fire still cannot be controlled, immediately carry out an emergency landing.

Carbon dioxide poisoning hazard study

After the disaster on October 24, 1947, as well as a similar incident of November 11 of that year, all DC-6s were grounded until the necessary changes were completed. However, these changes were so significant that the Civil Aeronautics Administration decided to re-certify DC-6. In January 1948, a series of flight tests was conducted to check the concentration of carbon dioxide in the cockpit, as in one of the tests the concentration reached values ​​sufficient for the crew to partially lose their legal capacity. Moreover, in this test there were official representatives of the Civil Aeronautics Administration and the Association of Civil Pilots, which is why they did not even try to hide this moment ^{[7] [8]} .

Then, additional safety valves were installed in the cabin under the floor and in the rear of the cab, which were supposed to enhance ventilation, thereby reducing the concentration of hazardous gas. After that, flight tests were conducted, during which the aircraft performed a descent from 20,000 feet (6,100 m ) at a speed of 300 knots, when at first one tank with carbon dioxide was completely empty in his cargo compartment, and after a while the second. According to the results of measurements, the concentration of carbon dioxide in the cabin was already 2%, which was considered satisfactory, after which on March 1, 1948 a certificate of completion of the tests was signed. However, in the tests, situations such as a decrease in the landing configuration (landing gear and flaps were released), that is, at a reduced speed, as well as the simultaneous opening of two tanks with carbon dioxide, were not worked out. The ventilation system did not change, only the procedure for extinguishing a fire was revised and the danger of using several bursts for extinguishing at once was indicated ^[8] .

On February 7, 1948, the Douglas Aircraft Company ordered medical studies on the effect of high concentrations of carbon dioxide on crew capacity. According to the results of which it was found that at a concentration of not more than 5%, which is the maximum, it is allowed to work no more than 5 minutes, since otherwise toxic poisoning begins, accompanied by irritation of the eyes, mucous membranes of the nose and mouth, muscle weakness, dizziness , coordination is lost action and may even faint . The results of these tests at the time of Mount Carmel’s disaster were still being processed and were not published. The Association of Pilots, in turn, issued a recommendation on March 3, 1948, that all members of the flight crew of civilian aircraft should have oxygen masks so that pilots can land the aircraft even with smoke in the cockpit or at dangerous concentrations of carbon dioxide. On April 13, the Association sent another letter to the Civil Aeronautics Administration stating the danger of carbon dioxide poisoning and the need for respirators in the cockpit, while the results of medical research on this issue were added to support the letter. At the time of the crash of Flight 624, these proposals were also just being considered ^[8] .

Insufficient knowledge of the question of what concentration of carbon dioxide is the limiting for long-term operation led to a branch of opinions about what concentration is limiting when flying at high altitudes. But the danger of such a situation was confirmed by the incident of May 13, 1948, when on board the Lockheed Constellation of Trans World Airlines , which was traveling at an altitude of 19,000 feet (5800 m ), there was a false warning about a fire in the front cargo compartment, so carbon dioxide fire extinguishers were activated. As a result, carbon dioxide entered the cockpit, where it reached such a concentration that some crew members lost their legal capacity, but fortunately still managed to make a safe emergency landing at a military airfield in Chillicote ( Missouri ). Information about this incident was brought to the Office of Civil Aeronautics and the Council for Civil Aviation , and on June 9 conducted flight tests at the Lockheed L-049 Constellation , which confirmed that the concentration of carbon dioxide in the air in this situation is so high that at least one crew member may lose legal capacity. Also, telegrams were sent to the operators of all modern aircraft, including DC-6, about the dangers of placing carbon dioxide fire extinguishers in the cargo compartments. A copy of this telegram was sent to the Douglas Aircraft Company, but from there they replied that, according to the results of tests from January of the same year, the ventilation on the DC-6 works at a satisfactory level, and therefore there should not be a repeat of the situation like on the Lockheed Constellation. This response was sent on June 15, two days before the disaster on board the NC37506 ^[9] .

Despite this response from Dugas, DC-6 pilots were advised that they should use oxygen masks when suspected of leaking carbon dioxide into the cockpit. At United, they were just developing instructions for their crews when the Mount Carmel disaster happened. After that, the Douglas Aircraft Company conducted 59 different tests, during which, when performing a descent from 20,000 feet (6,100 m ), the fire system was used in accordance with the instructions. It was found that with a decrease at a speed of 300 knots, the maximum concentration in the cabin reached 4.2%, and even that was only in one test, while for three minutes the average concentration of carbon dioxide was approximately 2%. If the aircraft was reduced at a speed of 160 knots with the landing gear and flaps extended, then if fire extinguishers were used according to the instructions, the peak concentration of carbon dioxide in the air reached already 7.8%, and the average for six minutes that the test lasted was 6.3%, only approximately at sea level, falling to 4.8% ^[4] .

Data Analysis

During his last radio communication, the crew of flight 624 announced the start of the emergency reduction, that is, he really believed that there was a fire on board, and therefore he clearly began to act according to the rules established for this situation, including activated carbon dioxide fire extinguishers. Although, by the way, a check of the debris showed that there might have been no fire, and the fire alarm was false. Then 10 minutes passed, during which the plane flew over the well-viewed Sunbury airfield, heading towards the mountains, where in principle a safe landing cannot be performed. This may mean one thing - the pilots lost their legal capacity due to the high concentration of carbon dioxide in the cockpit, which leaked there from the cargo compartment after activating the fire extinguishers. The safety valves for regulating the pressure with the passenger compartment were closed, which means that all the carbon dioxide from the cargo compartment went into the cabin ^[4] .

Nevertheless, when studying the wreckage, it was determined that the pilots did not actually complete the established procedures, since the safety valves were closed before the fire extinguishers were activated. The landing gear was removed during the descent, and the position of the flaps could not be precisely determined, but it is likely that the latter were still released. In this case, the crew would perform a reduction at a speed of not about 300 knots, with which certification tests were carried out, but almost half lower - 160 knots, at which, as later tests showed, even in the case of strictly following the instructions, the cabin would still form dangerous concentration of carbon dioxide ^{[4] [10]} .

Cause of the disaster

The cause of the disaster was the crew’s loss of legal capacity due to poisoning by a high concentration of carbon dioxide in the cockpit ^[10] .

The catastrophe on board the NC37506 led to significant advances in addressing fire safety issues and crew actions in this situation, including psychological aspects. The fire-fighting system of the DC-6 aircraft was also changed and the ventilation of the cabin and the cabin was strengthened. The order of actions of the crew in emergency situations also underwent changes. Now the pilots were obliged to use oxygen masks always in emergency situations, including smoke, or with an increase in carbon dioxide concentration ^[11] .

Comments

Sources

• UNITED AIR LINES, INC., NEAR MT. CARMEL, PENNSYLVANIA, JUNE 17, 1948 (inaccessible link) . Civil Aviation Council (2 August 1949). Date of treatment June 16, 2015. Archived January 29, 2016.

• ASN Aircraft accident Douglas DC-6 NC37506 Mount Carmel, PA . Aviation Safety Network . Date of treatment June 16, 2015.