American Trans Air Flight 406
Boeing 727-290 · Incident: decompression en route
From Chicago Midway International Airport (MDW) to St. Petersburg Clearwater International Airport (PIE)
Event
- NTSB case
- CHI96IA157
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Incident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 0 · all aircraft and ground
- Ground fatalities
- 0
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- Airline
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
Boeing 727-290
- Aircraft type
- Boeing 727
- Category
- Airplane
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N775AT
- Operator
- American Trans Air
- Onboard fatalities
- Unknown
- Route
- From Chicago Midway International Airport (MDW), Chicago, ILTo St. Petersburg Clearwater International Airport (PIE), St. Petersburg, FL
- Aircraft age
- Not recorded
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 121: Air Carrier
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- En route · cruise
- First occurrenceThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- Decompression
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- None
Cause areas
- Flight engineer › Miscellaneous equipment, improper use of
- Flightcrew › Checklist, not used
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
Upon reaching a cruise altitude of 33,000 feet, the cabin altitude warning horn sounded. The captain noticed the right air conditioning pack was off and he, along with the flight engineer, attempted to reinstate the pack without using a checklist. The cabin altitude continued to climb to 14,000 feet at which time the warning lights illuminated and the oxygen masks deployed in the cabin. While attempting to correct the cabin altitude, the flight engineer inadvertently opened the outflow valve resulting in a rapid loss of cabin pressure. The captain, the flight engineer, and the lead flight attendant all subsequently became unconscious due to hypoxia. The captain had delayed donning his oxygen mask. The flight engineer became unconscious after reviving the flight attendant. The first officer, who had only 10 hours of flight time in the airplane, had donned his oxygen mask when the warning horn first sounded, maintained consciousness, and was able to initiate an emergency descent. During the emergency descent the captain, the flight engineer, and the attendant regained consciousness, and an emergency landing was made at Indianapolis, Indiana. The airplane was inspected and flight tested the next day. The airplane's pressurization system functioned with no anomalies.
Probable cause
the failure of the captain and flight engineer to utilize a checklist to troubleshoot a pressurization system problem, and the flight engineer's improper control of the pressurization system which resulted in an inadvertent opening of the outflow valve and subsequent airplane decompression.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record CHI96IA157
- Event ID
- 20001208X05709
- Case number
- CHI96IA157
- Dataset
- historical-pre2008
- Source SHA-256
- 89e2df6d848e7ab3b42e7d7c7a03ff403303057a26ca04c95b3ea67201c3ce74
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: local date, investigation status, damage. Values present in the bulk record are kept.
- The date is the local date; the NTSB stores the UTC date 1996-05-13.
- API snapshot SHA-256: 0b284a7a99315b483af198018e810d707afd91f6c6aed30aab8c59b3be298606; retrieved 2026-09-18T17:51:17.465Z.