Blue B734 near Bacau on Aug 10th 2017, cabin did not pressurize

Last Update: March 13, 2023 / 17:47:56 GMT/Zulu time

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Incident Facts

Date of incident
Aug 10, 2017


Blue Air

Flight number

Aircraft Registration

Aircraft Type
Boeing 737-400

ICAO Type Designator

A Blue Air Boeing 737-400, registration YR-BAJ performing flight 0B-2931 from Bacau (Romania) to London Luton,EN (UK), was climbing out of Bacau when the crew stopped the climb at about FL200 due to the cabin not pressurizing, the passenger oxygen masks deployed, the aircraft performed an emergency descent to FL100 and returned to Bacau about 35 minutes after departure.

A replacement Boeing 737-400 registration YR-BAU reached Luton with a delay of 4:15 hours.

The occurrence aircraft returned to service about 14 hours after landing back and following positioning flights to Bucharest Otopeni and back to Bacau.

On Mar 13th 2023 Romania's AIAS released their final report concluding the probable cause of the serious incident was:

The serious incident occurred due to improper application of the checklist “CABIN ALTITUDE WARNING or Rapid depressurization” as result of a malfunction of the aircraft pressurization control system in the AUTOMATIC mode.

The AIAS analysed the occurrence:

The analysis of the audio recordings and the data obtained revealed the
following aspects and relevant moments in of the occurrence:

The crew performed the standard procedures during the taxi and take-off lineup

Take-off went normally, with the FO on the aircraft controls as the Pilot Flying (PF) and the pilot-in-command as the Pilot Monitoring (PM). The take-off is recorded at 05:40 UTC – minute 02:02 (approximately) on the CVR recordings, when the PF declared the initiation of specific maneuvers to perform take-off.

According to the QAR recordings, take-of took place at 05:40:47 UTC.

The first part of the climbing flight went normally; the crew operates according to the company’s standard procedures. The pilot-in-command (as the PM) announced the after take-off checks at minute 07:30 on the recording, immediately after passing through the flight level 100 (altitude 10.000 ft std).

When passing through the flight level 150 (minute 09:24 on the recording), FO made the standard announcement: „Flight level 150, climb thrust and cabin climbing”

At minute 12:20 (2 min. and 50 sec. after the last reference to the cabin altitude) FO informed the CPT that the „cabin climbs pretty fast …we should have had around 2 now and we have..9”. (Note:. The figures he refers to are thousands of feet of cabin altitude)

Note: the altitude in the cabin was already 9000 ft, instead of almost 2000 ft., in case of normal operation of the pressurization system.

According to the flight parameter recorded by QAR, the aircraft exceeded the altitude of 22.500 ft in climbing.

At min 13:10, CPT requires the FO to decrease the rate of climb and stop climbing.

At min 13:25 the aural warning for exceeding the maximum altitude in the cabin, namely 10,000 ft is triggered.

At min 13:30 CPT request to descent to 10.000 ft., QNH. The request was approved immediately, for flight level 150. The pilot-in-command descent to 10,000 ft, and the air traffic controller approves flight level 110, which was the minimum level en route in the area.

At min 13:35 Master Caution Signaling was triggered too.

Note: The Master Caution warning was triggered by the failure of the pressurization control in Automatic Mode (AUTO FAIL), caused by the excessive climbing rate of the cabin altitude. No remarks from the pilots regarding this occurrence can be heard in the audio recordings.

At min 13:57, 32 of seconds after the cabin altitude exceedance warning became active the crew began descending the aircraft to the approved level 110.

At min. 14:06 CPT can be heard breathing through the oxygen mask. 40 seconds had passed since the flashing warning for exceeding the cabin altitude of 10,000 ft became active

At min. 14:20 CPT selected the emergency code 7700 on the transponder, according to the „Emergency descent” procedure in the FCOM Manual.

At min 14:48 the intermittent aural warning signal of exceeding the altitude in the cabin became inactive (1 minute and 23 seconds from its triggering).

At min 15:28 CPT informs through the Passenger Address system (P/A) the passengers and flight attendants about „Emergency descent”; he informs them that they have to use the oxygen masks. He also puts on the announcement for passengers on using the oxygen masks, which starts playing in the speakers. This is most likely when he release the oxygen masks.

At min 15:55 the two pilots established the intercommunication through the oxygen masks. (2 minutes and 30 seconds after the cabin altitude flashing warning became active).

At min 16:22 CPT inform ATC about intention to return to Bacău. The request was approved immediately. The aircraft altitude was 17300 ft.

At min 19:01 FO informs: „the cabin is still not descending… it went down a little”.

At min 20:50 CPT informs the passengers about the decision to return to Bacău. At min. 23:10 CPT asks the FO to perform the “Emergency descent Checklist”.

Note: There were almost 10 minutes since the maximum cabin altitude warning
became active

At min. 23:40 while doing the Checklist, FO finds out that: „..mode selector….. MANUAL AC, …and…that…we didn’t do”

Note: MANUAL AC is the manual control procedure of the cabin pressurization
control system.

At min 24:35 FO finds out that: „ come on, I closed it and now it's going down..........; it’s going down too fast…”

Note: According to the procedure, he manually closed main outflow valve/ and the cabin altitude started to go down too fast.

At min 26:23 – while reading the Checklist – the crew finds that prior to initiating the emergency descent, the engine start switches should have been switched on „Continuous” position in order to avoid the „Flame out” phenomenon (flame break in the combustion chamber and the risk of uncontrolled engine shutdown) when suddenly reducing the engine power control levers during the emergency descent.

At min 34:48, during the final approach, CPT decides to take the flight control from FO.

The flight parameters (speed, altitude, vertical speed) were corroborated with the MASTER CAUTION indication. According to FCOM, the MASTER CAUTION signaling is triggered by the AUTO FAIL warning – failure of the cabin pressurization control automatic system and/or by the OFF SCHEDULLE DESCENT warning.

From the analysis, it appears that the crew did not perform the specific check-list to the abnormal/emergency situation that occurred.
Incident Facts

Date of incident
Aug 10, 2017


Blue Air

Flight number

Aircraft Registration

Aircraft Type
Boeing 737-400

ICAO Type Designator

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