The three terms describe how fast the cabin loses its pressure. The FAA’s high-altitude guidance, Advisory Circular 61-107B, defines an explosive decompression as a change in cabin pressure faster than the lungs can decompress, and says most authorities count anything under half a second. A rapid decompression is one where the lungs can empty faster than the cabin. A slow or gradual decompression may not be noticed at all, and the FAA calls it as dangerous as the other two, or more so.
Why airliners are pressurised
At cruising height the air is too thin to keep people conscious. The FAA’s guidance points out that at 18,000 feet the air has half its sea-level density, so each breath holds about half the oxygen. Airliners pump air into the cabin to keep it at a lower “cabin altitude”. US certification rules (14 CFR 25.841) require that, in normal operation, the cabin stays at a pressure altitude of no more than 8,000 feet.
The same rule covers failures. An airliner must be designed so that, after any decompression not shown to be extremely improbable, the people on board are not exposed to a cabin altitude above 25,000 feet for more than 2 minutes, or above 40,000 feet at all. Pressurised airliners also carry a cabin altitude warning that sounds when the cabin climbs past 10,000 feet.
Explosive, rapid and slow decompression: the difference
| Type | What happens (FAA AC 61-107B) | How it feels |
|---|---|---|
| Explosive | Cabin pressure drops faster than the lungs can empty, usually in under 0.5 seconds. More likely in small cabins than large ones, and can damage the lungs. | Loud noise, fog in the cabin, loose objects thrown about |
| Rapid | Cabin pressure drops quickly, but the lungs can empty faster than the cabin. Lung injury is much less likely. | Noise, fog, a sudden chill, ears and stomach expanding |
| Slow or gradual | Cabin pressure leaks away over minutes or hours. | Often nothing at all, until hypoxia sets in |
The size of the cabin matters. The FAA lists the cabin of a Boeing 747 as about 223 times the volume of a small Learjet’s. A hole of the same size will empty a small business jet far faster than a wide-body airliner.
What passengers experience, and why the masks matter
A fast decompression is hard to miss. The FAA lists noise, sudden fog as the air cools, and flying debris. Gas in the ears and gut expands. The passenger oxygen masks drop when the cabin altitude passes a set height, and the crew put on their own masks and start an emergency descent to below 10,000 feet.
Passenger masks are meant to bridge that descent. On Helios 522 they deployed as planned, but nobody descended. In a slow decompression the danger is that nobody recognises anything is wrong. The FAA warns that hypoxia comes on gradually and dulls the mind, and that euphoria, a false sense of well-being, is probably the most dangerous of its symptoms.
Time of useful consciousness
Time of useful consciousness is the time between losing oxygen and no longer being able to take the right action. The FAA publishes average values, and warns that they vary widely between people and that impairment can begin at once. The column on the right applies after a rapid decompression, when the body has no time to adjust.
| Altitude | Time of useful consciousness | After a rapid decompression |
|---|---|---|
| 18,000 ft | 20–30 minutes | 10–15 minutes |
| 22,000 ft | 10 minutes | 5–6 minutes |
| 25,000 ft | 3–5 minutes | 1.5–2.5 minutes |
| 28,000 ft | 2.5–3 minutes | 1–1.5 minutes |
| 30,000 ft | 1–2 minutes | 30 seconds–1 minute |
| 35,000 ft | 30 seconds–1 minute | 15–30 seconds |
| 40,000 ft | 15–20 seconds | nominal |
That is why pilots are trained to put their own masks on first, before they do anything else.
Aloha 243: a section of roof torn away
On 28 April 1988, Aloha Airlines Flight 243, a Boeing 737-200 built in 1969, was levelling at 24,000 feet between Hilo and Honolulu when about 18 feet of the upper fuselage separated, from behind the cockpit almost to the wings. Senior flight attendant Clarabelle “C.B.” Lansing was swept out of the aircraft and never found. The passengers were all seated with seat belts fastened. The crew made an emergency descent and landed at Kahului on Maui at 13:58, 13 minutes later. Of the 95 people on board, 94 survived. Eight were seriously injured. The 13 minutes to the landing are covered in Aloha Airlines Flight 243: 13 Minutes From Decompression to Maui.
The NTSB (report AAR-89/03) traced the failure to a lap joint on the left side of the fuselage. There, the bonding between the overlapping skin panels had come apart and fatigue cracks had grown from many rivet holes in a row. When the cracks linked up, the tear ran past the tear straps that were meant to stop it and let the cabin vent slowly. The aircraft had flown 89,680 flights. Each one was a full cycle of pressurising and relaxing the fuselage. The NTSB’s probable cause was the failure of Aloha’s maintenance programme to detect significant disbonding and fatigue damage, with company management and FAA oversight contributing.
Aloha 243 is the clearest case of a decompression caused by structural failure that started as a maintenance error. The inspection changes that followed are in Aloha 243 and the Aging Aircraft Rules That Followed It.
Helios 522: a slow, silent loss of pressure and hypoxia
Helios Airways Flight 522 is the opposite case. Nothing broke. On 14 August 2005 the Boeing 737-31S, 5B-DBY, took off from Larnaca for Athens and Prague with its pressurisation mode selector still in manual after a pressure test overnight. The outflow valve was partly open, so the cabin never pressurised as the aircraft climbed. See Helios Airways 522: The Switch, the Horn and Andreas Prodromou.
As the cabin passed 10,000 feet the warning horn sounded. On the 737, the cabin altitude warning used the same horn as the take-off configuration warning, which pilots heard on the ground in routine checks. The crew treated it as a configuration fault. Then an equipment cooling warning appeared, a side effect of the thin air, and the captain discussed it with the airline’s engineer by radio. According to the AAIASB report, the passenger oxygen masks deployed as the aircraft passed 18,200 feet. The pilots never put on their own masks, and hypoxia overcame them.
The autopilot levelled the aircraft at 34,000 feet and flew it to Athens and into a holding pattern. It flew on for almost three hours until the fuel ran out. It struck hills near Grammatiko at 12:03 local time, and all 121 people on board were killed. The AAIASB found the crew did not recognise that the selector was in manual and did not identify the warnings. It named the operator’s safety culture, regulatory oversight and the manufacturer’s response to earlier pressurisation incidents as latent causes. How the flight ended is covered in Planes That Ran Out of Fuel.
Other cases
Japan Air Lines Flight 123 now has its own event page; the other two do not yet.
- Japan Air Lines Flight 123, 12 August 1985. About 12 minutes after leaving Tokyo, the Boeing 747SR’s aft pressure bulkhead ruptured. According to the FAA’s summary of the Japanese investigation, the pressure surge tore away much of the vertical fin and severed all four hydraulic systems. Of the 524 people on board, four survived. The root cause was a repair to the bulkhead, made several years earlier and done incorrectly, which let fatigue cracks grow undetected.
- Southwest Airlines Flight 1380, 17 April 2018. A fan blade on the Boeing 737-700’s left engine separated because of a fatigue crack. Fragments of the engine cowling struck the fuselage and the cabin depressurised. One passenger was killed. The NTSB titled its report “Left Engine Failure and Subsequent Depressurization”.
- Sunjet Aviation Learjet 35, near Aberdeen, South Dakota, 25 October 1999. Radio contact was lost after the jet was cleared to 39,000 feet on a flight from Orlando to Dallas. It flew on until its fuel ran out, and all six people on board were killed. The NTSB found the crew were incapacitated because they did not receive supplemental oxygen after a loss of cabin pressure, for reasons it could not determine.
What changed
- Inspections counted in flights, not only years. Aloha 243 showed that a fuselage ages with every pressurisation cycle. The FAA started an ageing aircraft research programme, and Congress passed the Aging Aircraft Safety Act of 1991.
- Pressure bulkheads and repairs. After Japan Air Lines 123, the FAA amended its structural rule, 14 CFR 25.365, to cover a sudden depressurisation into smaller compartments. It also required repair assessment programmes for pressurised airliners flown beyond set numbers of cycles.
- A horn pilots cannot mistake. After Helios 522, the FAA required separate warning lights so pilots can tell which alert the horn means. Crews are now trained to treat a cabin altitude warning in flight as a signal to put on oxygen and descend at once.
- Pressurisation checklists. Boeing made returning the pressurisation selector to auto an explicit step after maintenance tests, and preflight checks now look at the selector itself, not only the altitude settings.
Each of these chains had more than one weak point: a missed crack, a misread horn, a skipped check. The Swiss cheese model explains why investigators look at all of them. The full findings are in the reports listed below, and How to Read an Air Accident Final Report shows how to find them.
The two live cases compared
| Aloha Airlines 243 | Helios Airways 522 | |
|---|---|---|
| Date | 28 April 1988 | 14 August 2005 |
| Aircraft | Boeing 737-200, built 1969 | Boeing 737-31S, built 1998 |
| Type of decompression | Explosive: about 18 feet of upper fuselage separated at 24,000 ft | Slow: the cabin never pressurised during the climb |
| Cause (official) | Undetected disbonding and fatigue cracking along a lap joint (NTSB) | Pressurisation mode selector left in manual; warnings not identified (AAIASB) |
| Warning | Unmistakable: wind, noise, open sky | Cabin altitude horn, mistaken for a take-off warning |
| Outcome | 94 of 95 survived; one flight attendant lost | All 121 on board killed |
Watch in the library
- Aloha Airlines Flight 243: Landing a 737 With Its Roof Torn Away
- Helios Airways Flight 522: How a 737 Flew On After Crew Hypoxia
- Japan Air Lines Flight 123: How the Bulkhead Burst Took the Tail
Every video on this subject is on the Structural failure page.
Sources
- Federal Aviation Administration: Advisory Circular 61-107B Change 1, Aircraft Operations at Altitudes Above 25,000 Feet MSL or Mach Numbers Greater Than .75 (types of decompression; table 2-3, times of useful consciousness)
- US Code of Federal Regulations (eCFR): 14 CFR 25.841, Pressurized cabins
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-89/03, Aloha Airlines Flight 243, Boeing 737-200, N73711, near Maui, Hawaii, April 28, 1988
- Air Accident Investigation and Aviation Safety Board (AAIASB), Greece: Aircraft Accident Report 11/2006, Helios Airways Flight HCY522, Boeing 737-31S, Grammatiko
- Federal Aviation Administration: Lessons Learned: Japan Airlines Flight 123, JA8119, Gunma Prefecture, August 12, 1985
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-19/03, Left Engine Failure and Subsequent Depressurization, Southwest Airlines Flight 1380
- National Transportation Safety Board: Aircraft Accident Brief NTSB/AAB-00/01, Sunjet Aviation Learjet 35, N47BA, Aberdeen, South Dakota, October 25, 1999
Last checked 31 October 2026.