A Link Airways flight from Canberra to Sydney in March had to reduce its height and slow down when the pilot realised one of the doors wasn't sealed tight.
The official aviation safety regulator report found air was escaping, with a loud whistling in the cabin as air got out through a faulty seal in the door. There were two pilots, a cabin manager and 32 passengers on board.
Australian Transport Safety Board investigators said: "The crew informed air traffic control of the pressurisation issue and requested clearance for a cruise altitude of 10,000ft (3048m). Clearance was granted, the aircraft was descended, and cabin altitude was maintained at 6500ft (1981m).
"To minimise passenger discomfort caused by the loud whistling, the flight crew elected to reduce the airspeed to a maximum of 200 knots (nautical miles per hour) for the remainder of the flight."
The aircraft normally cruises at 35,000ft (10,668m) and travels at 270 knots.
The problem was noticed by the pilot as the Saab 340B was over Goulburn.
The ATSB has not apportioned blame but said Link Airways had retrained staff so they could better identify faults.
The fault in this case was a seal on the door which had failed. Staff had noticed it the day before but assumed it was just a cosmetic issue rather than something fundamental (like the pressurisation system on the aircraft not working properly).
The incident, which happened on March 25 this year, follows an emergency landing in November at Canberra Airport by the same type of aircraft also operated by Link Airways.
In that earlier emergency, passengers screamed in horror when an object from the propeller pierced the fuselage as it took off. They then spent 10 minutes in terror in the air with the hole in the side of the aircraft. That incident is still being investigated by the ATSB.
A strap from the propeller had flown into the side of the aircraft as it accelerated down the runway. The strap then went through the casing and hit a woman in seat 2A, passengers immediately behind her said.
There is no indication in its report on the latest incident about what passengers felt as they heard the whistling noise and realised the plane was flying much lower than normal.
The faulty door seal was spotted by a flight crew the day before but they didn't think it was serious.
"Due to an incorrect assessment that the broken section was simply a piece of cosmetic trim, the off-going flight crew inappropriately applied the company's non-safety of flight defect deferral process in phone consultation with the continuing airworthiness maintenance organisation representative and a licensed aircraft maintenance engineer," the ATSB report said.
"Aircrew and maintenance engineers should remain vigilant of the potential for misunderstanding and use confirming techniques such as follow up questioning, demonstration, or the use of photos or video to ensure accurate and effective communication."
The investigation into the first, more serious, incident has been widened. The ATSB said that the investigation unit had identified "several risk controls with limitations that contributed to the occurrence. Examination of these factors represent a significant increase in the scope of this investigation".
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