Another De Havilland Canada Dash 8-400 suffered a rapid cabin depressurisation on 17 July, an incident with similarities to three recent depressurisations involving the type.
The previous incidents involved two Porter aircraft, including the same involved in the July depressurisation, and one operated by WestJet.
In January, following the three previous incidents, both Porter and WestJet told FlightGlobal they were making aircraft modifications to address a known depressurisation issue caused by a cargo door problem.
The latest incident involved a Porter Dash 8-400 (registration C-GLQO) operating flight 939 from Billy Bishop Toronto City airport to Boston on 17 July.
During the flight, when near Albany, New York, the "crew experienced a pressurisation issue" that caused the cabin altitude to climb "at a very high rate", says an incident report from the Transportation Safety Board of Canada (TSB).
The pilots initiated an emergency descent and declared mayday. When the aircraft descended to 10,000ft "it was determined there was no structural damage" and the aircraft continued safely to Boston, says the report.
"A post-flight inspection revealed the aft baggage blowout panel was loose," it adds.
The TSB's report does not mention the cargo door issue that caused previous Dash 8-400 depressurisations, and neither the TSB nor Canadian regulator Transport Canada immediately responded to requests for more information.
However, the latest incident shares similarities to the previous depressurisations.
Those incidents include depressurisations of the same Porter aircraft in August 2018, another Porter Dash 8-400 (registration C-GLQN) in November 2018 and a WestJet Encore aircraft (registration C-FWEP) in January 2019.
Earlier TSB reports said pilots of those aircraft received fuselage door warnings prior to depressurisations.
Workers later traced those incidents to cargo door handles being either improperly secured or "moving out of the flushed, latched position".
As a result, the cargo compartments depressurised, which, in one instance, caused the "aft cabin bulkhead blowout panel [to] became dislodged", the reports said.
Porter did not immediately respond to a request for comment about the latest incident, but last year told FlightGlobal the incidents were "an operational inconvenience more than anything else".
Then in January Porter and WestJet said they were making modifications to address the issue. They said the previous owner of the Dash 8 programme, Bombardier, had issued guidance for a fix.
"We are in the process of modifying the door handles as outlined in the recommendation. The work began this month," Porter said in January. "The work will be completed across our 29 aircraft during scheduled maintenance intervals in the coming weeks."
De Havilland Canada, which acquired the Dash 8-400 programme from Bombardier in June, did not immediately respond to a request for comment.
In January WestJet noted that Transport Canada had "determined that there is no risk to safety and the decision to complete the modification is at each company’s discretion".
Story updated on 2 August with the below comments from Porter and Transport Canada.
Porter tells FlightGlobal on 2 August that the 17 July depressurisation "is different than past circumstances... involving the cargo door handle".
The recent event "related to an imperfection in the door seal, which caused the panel to move", Porter says. "The system worked as intended to equalise pressure in different areas of the aircraft, and the flight continued to Boston at a lower altitude and without further issue".
"Porter has a proactive programme to replace door seals, although there is no required time frame for doing so," it adds. "The failure rate is extremely low."
Additionally, Porter says its effort to modify door handles "is ongoing", noting there is no regulatory requirement to perform the work.
"We will continue with the programme in the coming months during extended scheduled maintenance," it adds.
Transport Canada tells FlightGlobal on 2 August that it is "aware of the recent occurrence on a Porter Airlines Q400 and is working with the manufacturer to determine if an unsafe condition is present".