Redegørelse 2025-182

Introduction

This bulletin reflects the opinion of the Danish Accident Investigation Board regarding the circumstances of the occurrence and its causes and consequences.

In accordance with the provisions of EU Regulation 996/2010, the Danish Air Navigation Act and pursuant to Annex 13 of the International Civil Aviation Convention, the safety investigation is of an exclusively technical and operational nature, and its objective is not the assignment of blame or liability.

The safety investigation was carried out without having necessarily used legal evidence procedures and with no other basic aim than preventing future accidents and serious incidents.

Consequently, any use of this bulletin for purposes other than preventing future accidents and serious incidents may lead to erroneous or misleading interpretations.

A reprint with source reference may be published without specific permission.

General

State file number:	2025-182
UTC date:	6-4-2025
UTC time:	16:55
Occurrence class:	Serious incident
Location:	Overhead the Bay of Biscay
Injury level:	None
Aircraft registration:	OY-JZK
Aircraft make/model:	B737-800 (86Q)
Current flight rules:	Instrument Flight Rules (IFR)
Operation type:	Charter
Operated by:	Jettime
Flight phase:	En route
Aircraft category:	Fixed wing
Last departure point:	Tenerife South (GCTS)
Planned destination:	Aalborg (EKYT)
Aircraft damage:	None
Engine make/model:	2 x CFM International CFM56-7B26

Synopsis

Notification

All time references in this bulletin are Coordinated Universal Time (UTC).

The operator notified the Aviation Unit of the Danish Accident Investigation Board (AIB) about the serious incident on 6-4-2025 at 19:30 hours (hrs).

The AIB notified the Danish Civil Aviation and Railway Authority (DCARA), the US National Transportation Safety Board (NTSB), the European Aviation Safety Agency (EASA), the International Civil Aviation Organization (ICAO) and the Directorate-General for Mobility and Transport (DG MOVE) on 6-4-2025 at 21:39 hrs.

The NTSB accredited a non-traveling representative to the AIB safety investigation.

Summary

During cruise overhead the Bay of Biscay, fumes and smoke emerged in the aft galley from a capacitor in oven #3. The cause of the fumes and smoke was aging of the capacitor.

A delay in disconnection of electrical power supply to oven #3 likely aggravated the development of fumes and smoke.

The electrical fumes entered the cabin. The flight and cabin crew decided as a precaution to discharge a halon fire extinguisher. The electrical fumes dissipated, and the flight crew decided to continue the flight.

Approximately 15 minutes after the halon was discharged, the electrical fumes reappeared. The flight crew decided to divert to Brest Bretagne (LFRB) and landed safely.

The safety investigation resulted in safety actions from the operator and the oven Original Equipment Manufacturer (OEM).

The serious incident occurred in daylight and under Visual Meteorological Conditions (VMC).

Factual information

History of flight

The serious incident occurred during a charter flight from Tenerife South (GCTS) to Aalborg (EKYT). The commander was Pilot Monitoring (PM), and the first officer was Pilot Flying (PF).

During cruise north of Spain overhead the Bay of Biscay, the cabin crew observed smoke and a smell of electrical fumes in the aft galley. The cabin crew incorrectly identified the origin of the smoke and fumes as coming from oven #4 (actual origin was oven #3). The cabin crew pulled the circuit breaker for oven #4 and reported the smoke and fumes to the flight crew.

Approximately four minutes later, the cabin crew pulled the circuit breakers for all the aft galley ovens. The commander instructed the cabin purser to observe the oven every fifth minute.

Fifteen minutes after the initial smoke and fumes observation, the electrical fumes were noticeable from the aft galley to the aft part of the cabin. This caused concern among passengers.

The commander consulted the first officer and the cabin purser before instructing the cabin purser to discharge a halon fire extinguisher. The cabin crew were uncertain about whether the smoke originated from oven #3 or #4.

The cabin crew initially struggled to break the seal on the halon fire extinguisher, but eventually managed to break it. The aft galley was cleared for all other personnel, and a cabin crew member fitted Portable Breathing Equipment (PBE/smoke hood) before discharging approximately half the content of a halon fire extinguisher into the area above oven #3 and #4.

The flight crew consulted the “Smoke, Fire or Fumes” emergency/Non-Normal Checklist (see Appendix 1) to item 10 “RECIRC FAN switches (both)… OFF” (without performing any of the items). The flight crew concluded that the checklist was dedicated to the cockpit only. The flight crew considered a diversion and discussed nearest suitable aerodromes.

The cabin purser reported to the commander that the electrical smell had dissipated and that there was no sign of smoke - only a slight smell of halon. The commander instructed the cabin purser to have a cabin crew member monitor the ovens in the aft galley.

The commander, first officer and cabin crew agreed that it was safe to continue the flight. The commander briefed the passengers on the situation and the mitigative measures implemented. The cabin purser informed the commander that all smell was gone.

The flight crew re-evaluated their decision to continue the flight versus making a diversion due to the use of the halon. While re-evaluating, the cabin purser reported the electrical smell reappearing, and the flight crew decided to divert to Brest Bretagne (LFRB).

The commander issued an urgency call (PAN-PAN) requesting vectors for LFRB. The Air Traffic Control Officer (ATCO) instructed the flight crew to squawk 7700 (distress/emergency) inbound LFRB. The cabin crew prepared the cabin for landing, and the commander briefed the passengers.

The ATCO cleared the aircraft for an Instrument Landing System (ILS) approach for runway 25L. The flight crew completed the relevant checklists and briefings, and the aircraft landed safely.

Aerodrome firefighting services inspected the aircraft on a remote parking stand with no findings.

Injuries to persons

Injuries	Crew	Passengers	Others
Fatal
Serious
Minor/None	6	185

Damage to aircraft

None.

Other damage

None.

Personnel information

License and medical certificate - the commander

The commander - male, 52 years - was the holder of a valid Airline Transport Pilot License (ATPL (A)).

The rating B737 300-900/IR was valid until 30-6-2025.

The medical certificate (class 1) was valid until 1-11-2025.

Flying experience - the commander

	Latest 24 hours	Latest 90 days	Total
All types	-	99	13,499
This type	-	99	4,519
Landings this type	-	20	1,490

License and medical certificate - the first officer

The first officer - female, 39 years - was the holder of a valid Commercial Pilot Licence (CPL (A)).

The rating B737 300-900/IR was valid until 28-2-2026.

The medical certificate (class 1) was valid until 6-12-2025.

Flying experience - the first officer

	Latest 24 hours	Latest 90 days	Total
All types	5:16	109	4,556
This type	5:16	109	2,708
Landings this type	1	11	543

Aircraft information

General information

Manufacturer:	The Boeing Company
Type:	B737-800 (86Q)
Serial number:	30296
Airworthiness review certificate:	Valid until 21-12-2025
Engine manufacturer:	CFM International
Engine type:	CFM56-7B26

Electrical power

The following text are extracts from the Flight Crew Operation Manual (FCOM) 1.40.73.

Electricity for the galleys was 115 Volt AC supplied from the airplane transfer buses and controlled by a switch on the overhead panel.

Cabin/Utility switch encircled red (FCOM 6.10.4) Figure 1. Cabin/Utility switch encircled red (FCOM 6.10.4)

When selected OFF, the CAB/UTIL switch removed electrical power from galley and cabin equipment systems including all 115 Volt AC galley busses and left & right recirculation fans.

Cabin crew oven firefighting procedure

The following text is an extract from the operator’s Operations Manual Part E (OM-E) section 2.14.1.4.:

Oven fires are usually caused by accumulated grease inside the oven. However, sometimes an oven fire can be caused by electrical problems at the back of the oven. That is why it is important to locate the source of the fire. Be aware that the primary hazard from an oven fire occurs when the door is opened. The introduction of oxygen can cause a flash fire.

Switch off electrical power to remove the heat source;
Close the oven door or keep the door closed. In most instances the fire will self-extinguish;
Monitor the situation having collected the BCF [halon] and the fire protection gloves (PBE should be within reach);
Monitor the situation;
Leave it closed for a short while and then with a BCF ready open the oven door to check that the fire has been extinguished;
Repeat the procedure if necessary;
Check the contents which have been on fire for any signs of smoldering and place in a leak proof container, ex. metal waste container;
Check surrounding areas for signs of heat and smoke;
If you do not expect the fire to be inside the oven, it might be an electrical fire. Discharge the BCF round the outside of the oven along the gap to reach the back of the oven where the electric cables are;
Check and guard the fire areas;
Do not use the oven for the remainder of the flight.

Meteorological information for Brest Bretagne (LFRB)

Terminal Aerodrome Forecast (TAF)

TAF LFRB

061100Z 0612/0718 VRB05KT 5000 BR OVC005 BECMG 0612/0614 SCT020 BECMG 0614/0616 04010KT TEMPO 0623/0709 3000 BR PROB40 TEMPO 0700/0708 0400 FG VV/// BECMG 0702/0704 VRB05KT TEMPO 0715/0718 06010KT=

Aviation Routine Weather Report (METAR)

METAR LFRB	061630Z AUTO 03008KT 350V060 CAVOK 17/10 Q1018 NOSIG=
METAR LFRB	061700Z AUTO 03010KT CAVOK 16/09 Q1018 NOSIG=
METAR LFRB	061730Z AUTO 04009KT CAVOK 15/09 Q1018 NOSIG=

Communication

The Air Traffic Control (ATC) communication was recorded on the Cockpit Voice Recorder (CVR).

Aerodrome information for LFRB

General information

Aerodrome Reference Point:	48º26’50’’N 004º25’18’’W
Elevation:	325 feet (ft)
Main runway directions:	07R/25L
Main runway dimensions	3,100 meters (m) x 45 m
Runway surface:	Asphalt

Flight recorders

Cockpit Voice Recorder (CVR)

The operator downloaded the audio data from the CVR and sent a digital copy to the AIB. The recording contained five channels of up to 120 minutes duration. The recording started at the initial part of the flight, and included the sequence of events. The CVR data was of good quality and useful to the AIB safety investigation.

Flight Data Recorder (FDR)

The operator collected FDR data for their Flight Data Monitoring (FDM) programme. The serious incident flight was recorded, and the operator sent a digital copy of the raw FDR data. The FDR data was not used in the AIB safety investigation.

Fire

There was no open fire. The operator and the oven Original Equipment Manufacturer (OEM) concluded the source of fumes/smoke to be a blown/burning capacitor inside oven #3. See Technical safety investigation.

Technical safety investigation

Similar occurrences on the aircraft

On 8-4-2025 (two days after the serious incident), the aircraft was released to service.

On 12-4-2025 (six days after the serious incident), the aircraft was operated on a flight from Billund (EKBI) to Chania (LGSA) with replaced ovens installed. The installed part numbers were slightly different from those involved in the serious incident.

During cruise, cabin crew members detected a smell of burned electrical wiring coming from one of the ovens in the aft galley. They pulled all oven circuit breakers, and the smell dissipated. This allowed the cabin crew to identify oven #2 (Serial number (S/N) 1000) as the source of the fumes.

The flight crew decided to continue the flight to LGSA. Upon arrival, the ovens in the aft galley were disconnected, and the return flight to Copenhagen (EKCH) was completed without any further occurrences.

After arriving at the operator base in EKCH, technicians removed oven #2, and the oven was included in the safety investigation.

Examinations of ovens

Operator technicians removed cover plates and inspected the electronics in two ovens from the serious incident flight and one oven (S/N 1000) from the flight on 12-4-2025 (six days after the serious incident).

Aft galley oven #3:	S/N 28054. Manufactured March 1998. (Source of smell/smoke on the serious incident flight 6-4-2025)
Aft galley oven #2:	S/N 0125, Manufactured May 2005. (Removed after the serious incident flight 6-4-2025 due to identification of internal burn marks)
Aft galley oven #2:	S/N 1000. Manufactured February 2004. (Source of smoke/fumes on the flight six days later)

The three ovens were shipped to the oven OEM for examinations. A representative from the Federal Aviation Administration (FAA) supervised the examinations.

On all three ovens, the oven OEM identified the source of fumes and smoke to be a blown capacitor on the power supply Printed Circuit Board (PCB). The purpose of the capacitor was filtering of electromagnetic noise generated by the oven. All three ovens were functional despite the capacitors failing. During the functional tests of the ovens, the
capacitors no longer emitted smoke or fumes.

Failed capacitors on the oven power supply PCBs Figure 2. Failed capacitors on the oven power supply PCBs

The capacitors were clear or silver like coloured when new, but turned yellow and bulgy with age. The oven OEM concluded that aging was the cause for the failing of the capacitors and the emissions of fumes and smoke. The oven OEM also concluded that the small amount of material in the capacitor would self-extinguish and be contained by the oven metal housing.

Since 2008, the oven OEM had used a higher rated capacitor for the power supply PCBs, which the oven OEM considered to be more resistant to failure. A 20-year life of the PCB (including the new type) was considered by the OEM to be adequate to prevent failures.

In addition, the oven OEM concluded that storage (more than six months) of the ovens could also result in degradation of the capacitors.

Analysis

General

The flight and cabin crew were licensed for the flight.

The aircraft was airworthy.

Technical cause of smoke and fumes

The oven OEM examined the ovens involved in the two events, and identified the source of fumes and smoke to be blown capacitors on the oven power supply PCB. All of the blown capacitors had been in service since pre-2008, which contributed to degradation by aging of the capacitors.

The oven OEM also concluded that storage of ovens could contribute to degrading of the capacitors.

Since 2008, the oven OEM used a higher rated capacitor version on the power supply PCBs. Despite being more resistant to failing, the oven OEM also considered the newer capacitor type prone to failing by aging, and therefore considered a 20-year life of the PCB as adequate to prevent failures.

Despite emitting smoke and harsh fumes, the blown capacitors did not have the potential to start a fire in the oven electronics. The metal casing of the oven was capable of containing the blown capacitor.

Flight and cabin crew handling

To mitigate electrical smoke, fumes or fire, the disconnection of the electrical power supply is essential. During the event, oven #4 was initially incorrectly identified as the source of smoke and fumes.

It took approximately four minutes before the circuit breaker for oven #3 (and all ovens in aft galley) was pulled by the cabin crew. This delay likely aggravated the amount of smoke and fumes.

The flight crew had the option of disconnecting all electrical power to the galleys by use of the Cab/Util switch on the cockpit overhead panel. This was part of the “Smoke, Fire or Fumes” Non-Normal Checklist (item 7, see Appendix 1). This option was not considered by the flight crew. The flight crew incorrectly concluded that the checklist was only applicable to fire, smoke and fumes events in the cockpit.

The decision to discharge a halon fire extinguisher was justifiable and in accordance with the OM-E procedure since the source of the fumes could not be surely concluded to originate from an oven. Discharge of halon would normally also involve an immediate diversion which was also constantly considered by the flight crew. Fumes from ovens are however not unusual and will not often result in a diversion.

The cabin crew struggled to break the seal on the halon fire extinguisher which can be fatal in case of an actual fire.

Since the fumes reappeared, and the exact cause of the electrical fumes could not be firmly concluded to be isolated to the oven during the flight, the flight crew decision to divert and land as soon as possible is considered prudent.

Conclusions

Summary

During cruise overhead the Bay of Biscay, fumes and smoke emerged in the aft galley from a capacitor in oven #3. The cause of the fumes and smoke was aging of the capacitor.

A delay in disconnection of electrical power supply to oven #3 likely aggravated the development of fumes and smoke.

Approximately 15 minutes after the halon was discharged, the electrical fumes reappeared. The flight crew decided to divert to Brest Bretagne (LFRB) and landed safely.

Safety actions

Based on the safety investigation, the following safety actions were implemented by parties involved in the safety investigation.

The oven OEM

The oven OEM revised the Component Maintenance Manual (CMM) to recommend:

- replacement of power supply PCBs manufactured before 2008;

- replacement of power supply PCBs older than 20 years;

- a functional check followed by inspection of PCBs prior to entering service when an oven has been stored for more than six months.

The oven OEM published a Service Information Letter (SIL) regarding inspection of the power supply PCB capacitor at shop visits, to makes sure the newer type capacitor is installed.

The operator

The operator initiated a campaign to replace of all the ovens in the fleet to a newer type.

The operator included the use of the “Smoke, Fire or Fumes” Non-Normal Checklist in flight crew recurrent training.

The cabin crew training was adjusted to include hands-on training in the use of fire extinguishers.

The operator revised the fire-fighting procedures for the cabin crew, so all circuit breakers shall be pulled in the affected galley, to remove electrical power from the source of fire as soon as possible.