Boeing 707 Involuntary De-Compression - ASD Report
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Boeing 707 Involuntary De-Compression - ASD Report
Another incident from Aviation Safety Digest, March, 1964 with its lessons still applicable today.
Just after commencing descent from 38,000 feet into Singapore a Boeing 707 suffered an involuntary de-compression. The cause of the de-compression has not been positively determined but is thought to have been a faulty outflow valve.
This was not a rapid de-compression – in fact the first indication of de-compression was an instrument noted increase of cabin altitude coincident with the Captain experiencing pressure effect on his ear drums. As cabin altitude continued to rise at a rate in excess of 2,000 feet per minute, de-compression was recognised and appropriate de-compression drill implemented.
As far as the cockpit aspects were concerned everything went smoothly. The operating crew were able to don their “quick-donning” (sweep-on type) masks and have them fully functioning within an estimated three seconds after positive recognition of de-compression; the aural and visual de-compression warnings operated at the appropriate times, and the aircraft was quickly established in rapid descent. There was no fogging.
On this occasion the events in the cabin were of greater interest. The Boeing 707 is fitted with passenger oxygen masks stowed in enclosures above each row of seats, so designed that a trap door automatically opens and permits the masks to drop out, ready for use, when the cabin altitude rises above 14,000 feet. The operator concerned makes a practice of briefing the passengers on the operation of the system prior to take-off.
The automatic release of the masks was the first indication in the cabin that a de-compression situation existed (remember that the aircraft had already commenced descent into Singapore). The galley warning light was not noticed.
A steward, on duty in the cabin, experienced pressure on his ears and slight breathlessness immediately before the masks fell. He put on a spare passenger mask and then checked the passengers, and found that most had their masks in position. He subsequently walked through the economy class cabin, with the cabin altitude at about 18,000 feet, without using a walk around oxygen set.
A second steward was in the galley. He saw the masks deploy and initially assumed that the mask retaining system had malfunctioned. He walked part way into the cabin and then returned to the galley to secure equipment against the steep descent. Within 30 seconds he felt nausea and was conscious of a lack of co-ordination and was later under the impression that he had knocked over several bottles and an ice-bucket. While this steward was under the impression that he knocked over the ice-bucket he was in fact observed to have lifted it and inverted it. Also he was seen to search for a key which he was holding between his teeth.
A third steward was in a toilet when the mask at that location was released. He also assumed a mask malfunction and endeavoured to re-stow the mask. He subsequently appreciated the true situation and moved though the cabin using the spare passenger masks as he moved along. Some passengers advised this steward that they did not feel the need to use a mask and did not do so.
With the exception of the one steward located in the galley, there was no significant hypoxic effect on cabin crew or passengers and it might well be asked what lessons can be learnt under these circumstances. Here are a few which could be very relevant to a future incident, particularly one involving a higher rate of de-compression.
The operator is examining means of providing a more effective cabin warning system.
Passenger briefing did not previously include a demonstration of the emergency masks. The operator has now elected to include an actual demonstration of the mask in the passenger briefing.
When masks release the first assumption should be that a de-compression situation exists and action should be taken on this basis until established otherwise.
Cabin crew members should use spare passenger masks or portable masks at all possible times and particularly when moving. They should not assume that they are all right – a false feeling of well-being is a common symptom of hypoxia.
For the same reasons as above, the average passenger is not fully capable of determining whether he is suffering or is likely to suffer hypoxia. Therefore every endeavour should be made to have every passenger breathe oxygen while the cabin is at an altitude where hypoxic effects are likely.
Just after commencing descent from 38,000 feet into Singapore a Boeing 707 suffered an involuntary de-compression. The cause of the de-compression has not been positively determined but is thought to have been a faulty outflow valve.
This was not a rapid de-compression – in fact the first indication of de-compression was an instrument noted increase of cabin altitude coincident with the Captain experiencing pressure effect on his ear drums. As cabin altitude continued to rise at a rate in excess of 2,000 feet per minute, de-compression was recognised and appropriate de-compression drill implemented.
As far as the cockpit aspects were concerned everything went smoothly. The operating crew were able to don their “quick-donning” (sweep-on type) masks and have them fully functioning within an estimated three seconds after positive recognition of de-compression; the aural and visual de-compression warnings operated at the appropriate times, and the aircraft was quickly established in rapid descent. There was no fogging.
On this occasion the events in the cabin were of greater interest. The Boeing 707 is fitted with passenger oxygen masks stowed in enclosures above each row of seats, so designed that a trap door automatically opens and permits the masks to drop out, ready for use, when the cabin altitude rises above 14,000 feet. The operator concerned makes a practice of briefing the passengers on the operation of the system prior to take-off.
The automatic release of the masks was the first indication in the cabin that a de-compression situation existed (remember that the aircraft had already commenced descent into Singapore). The galley warning light was not noticed.
A steward, on duty in the cabin, experienced pressure on his ears and slight breathlessness immediately before the masks fell. He put on a spare passenger mask and then checked the passengers, and found that most had their masks in position. He subsequently walked through the economy class cabin, with the cabin altitude at about 18,000 feet, without using a walk around oxygen set.
A second steward was in the galley. He saw the masks deploy and initially assumed that the mask retaining system had malfunctioned. He walked part way into the cabin and then returned to the galley to secure equipment against the steep descent. Within 30 seconds he felt nausea and was conscious of a lack of co-ordination and was later under the impression that he had knocked over several bottles and an ice-bucket. While this steward was under the impression that he knocked over the ice-bucket he was in fact observed to have lifted it and inverted it. Also he was seen to search for a key which he was holding between his teeth.
A third steward was in a toilet when the mask at that location was released. He also assumed a mask malfunction and endeavoured to re-stow the mask. He subsequently appreciated the true situation and moved though the cabin using the spare passenger masks as he moved along. Some passengers advised this steward that they did not feel the need to use a mask and did not do so.
With the exception of the one steward located in the galley, there was no significant hypoxic effect on cabin crew or passengers and it might well be asked what lessons can be learnt under these circumstances. Here are a few which could be very relevant to a future incident, particularly one involving a higher rate of de-compression.
The operator is examining means of providing a more effective cabin warning system.
Passenger briefing did not previously include a demonstration of the emergency masks. The operator has now elected to include an actual demonstration of the mask in the passenger briefing.
When masks release the first assumption should be that a de-compression situation exists and action should be taken on this basis until established otherwise.
Cabin crew members should use spare passenger masks or portable masks at all possible times and particularly when moving. They should not assume that they are all right – a false feeling of well-being is a common symptom of hypoxia.
For the same reasons as above, the average passenger is not fully capable of determining whether he is suffering or is likely to suffer hypoxia. Therefore every endeavour should be made to have every passenger breathe oxygen while the cabin is at an altitude where hypoxic effects are likely.
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Involuntary De-compression.
Interesting item Centaurus. The B707 used the output of three turbo-compressors driven by engine bleed air, one on each of engines 2,3 & 4, which compressed ambient air for air-conditioning and pressurisation. Usual procedure was to initially reduce thrust on engines 1 & 4 on descent, later reducing thrust on engines 2 & 3. If speed was allowed to reduce too much with all four engines back at low thrust settings the ram effect on the turbo-compressors would be lost thus reducing the volume of air entering the aircraft which, if not noted and corrected, could lead to the cabin altitude rising. You mentioned that the cause was not determined in the report cited. Just wonder if this possible cause was investigated.
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The early series B707 had the pressurisation exhaust via outflow valves which were subject to huge quantities of nicotine build up. This resulted in the valves being very slow to operate or on some occasions jam altogether. With a jamming valve and reduced pressure from the TCs on descent, things could get a bit difficult in maintaining the cabin pressure.
Later 707s had an addition to the system in thrust recovery valves which were venturis angled as thrust augmenters which took out the majority of the cabin air, the outflow valves did the finer adjustment. With this system an additional result was the outflow valve nicotine build up was substantially reduced and valve problems were reduced.
Wunwing
Later 707s had an addition to the system in thrust recovery valves which were venturis angled as thrust augmenters which took out the majority of the cabin air, the outflow valves did the finer adjustment. With this system an additional result was the outflow valve nicotine build up was substantially reduced and valve problems were reduced.
Wunwing
Join Date: Feb 2008
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Involuntary De-compression.
The early series B707 had the pressurisation exhaust via outflow valves which were subject to huge quantities of nicotine build up. This resulted in the valves being very slow to operate or on some occasions jam altogether. With a jamming valve and reduced pressure from the TCs on descent, things could get a bit difficult in maintaining the cabin pressure.
Later 707s had an addition to the system in thrust recovery valves which were venturis angled as thrust augmenters which took out the majority of the cabin air, the outflow valves did the finer adjustment. With this system an additional result was the outflow valve nicotine build up was substantially reduced and valve problems were reduced.
Wunwing
Later 707s had an addition to the system in thrust recovery valves which were venturis angled as thrust augmenters which took out the majority of the cabin air, the outflow valves did the finer adjustment. With this system an additional result was the outflow valve nicotine build up was substantially reduced and valve problems were reduced.
Wunwing
With the abolition of smoking being permitted the tar build up on the outflow valves vanished. Prior to banning smoking on aircraft the tar build up was starkly shown when comparing a pax aircraft to a freighter. I took a couple of photographs of both and gave them to a medical practitioner who was interested, back in the day.
