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
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State file number: |
2025-247 |
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UTC date: |
29-4-2025 |
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UTC time: |
18:52 |
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Occurrence class: |
Serious incident |
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Location: |
Aalborg (EKYT) |
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Injury level: |
None |
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Aircraft registration: |
OY-FHD |
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Aircraft make/model: |
De Havilland Aircraft of Canada DHC-6 Series 300 (Twin Otter) |
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Current flight rules: |
Visual Flight Rules (VFR) |
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Operation type: |
Instructional |
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Operated by: |
Zimex Aviation |
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Flight phase: |
Take off |
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Aircraft category: |
Fixed wing |
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Last departure point: |
EKYT |
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Planned destination: |
Roskilde (EKRK) |
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Aircraft damage: |
None |
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Engine make/model: |
2 x Pratt & Whitney Canada PT6A-27 |
Synopsis
Notification
The Safety Manager from the Danish Home Guard (Flyverhjemmeværnet) notified the Aviation Unit of the Danish Accident Investigation Board (AIB) of the serious incident on 8-5-2025 at 07:53 hours (hrs). The Danish Home Guard notified the AIB, because the serious incident involved a Danish Home Guard aircraft and a Danish Home Guard pilot undergoing training by Zimex Aviation.
The AIB notified the Danish Civil Aviation and Railway Authority (DCARA), the Transportation Safety Board of Canada (TSB), the Swiss Transportation Safety Investigation Board (STSB), the European Aviation Safety Agency (EASA), the Directorate-General for Mobility and Transport (DG MOVE) and the International Civil Aviation Organization (ICAO) on 12-5-2025 at 12:20 hrs.
All time references in this bulletin are Coordinated Universal Time (UTC).
Summary
During a takeoff ground roll when the student pilot applied full power, after the power had been in idle, the LH engine had flamed out. The instructor pulled back both throttle levers to idle and subsequently the RH engine flamed out.
Aircraft inspections and engine ground run tests indicated that the reason for the engine flameouts might be located to the Fuel Control Units (FCUs) and/or the High Pressure (HP) fuel pumps.
The engine manufacturer performed bench testing and tear down inspections of the FCUs and the HP fuel pumps without any findings, which could explain the engine flameouts.
The AIB safety investigation was unable to determine the cause of the engine flameouts.
Continuous ignition is applied on some aircraft types during critical phases of flight as a general mitigation against flameout risk, however, no causal relationship has been identified in this occurrence, and no safety recommendations are issued.
This serious incident occurred in daylight and under Visual Meteorological Conditions (VMC).
Factual information
History of flight
The flight was a VFR school flight from Aalborg (EKYT) to Roskilde (EKRK). The purpose was difference training of a student pilot holding a DHC6-400 Twin Otter type rating.
Prior to the incident, the instructor and student pilot completed seven takeoffs and landings on runway 26R at EKYT.
During the eighth takeoff ground roll, the instructor called “Stop, stop, stop” (to simulate a rejected takeoff), and the student pilot pulled the throttle levers back to idle. The instructor then called “Continue the takeoff”, and the student pilot applied partial power, checked all engine parameters were normal, and then applied full takeoff power.
The instructor monitored the engine parameters and noticed the LH engine torque and fuel flow indications dropped at an indicated airspeed of 50-60 knots (kt) and concluded that the LH engine had flamed out.
The instructor took control of the aircraft and pulled the throttle levers back to idle. He then noticed that the RH engine flamed out.
The student pilot and the instructor verified that fuel levers, fuel shut off valves and fuel pumps were set to the correct positions. The ignition switch was in the NORMAL position.
The instructor informed Air Traffic Control (ATC) of the situation restarted the LH engine and vacated the runway.
On the taxiway, the instructor restarted the RH engine. With both engines running and normal engine indications, the instructor tried to reproduce the flameouts by applying full takeoff power and then pulling the throttle levers back to idle. This was done twice satisfactory. During the third attempt, when the throttle levers were pulled back to idle, the RH engine flamed out and the LH engine continued running in idle.
The instructor taxied the aircraft back to the maintenance hangar.
Injuries to persons
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Injuries |
Crew |
Passengers |
Others |
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Fatal |
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|
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Serious |
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|
|
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None |
2 |
1 |
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Damage to aircraft
None.
Other damage
None.
Personnel information
License and medical certificate
The instructor - male, 58 years - was the holder of a valid Commercial Pilot Licence (CPL/(A)) issued by the Swiss Federal Office of Civil Aviation (FOCA).
The DHC-6 type rating and Instrument Rating (IR(A)) were valid until 28-2-2026.
The Flight Instructor rating (FI(A)) DHC-6 was valid until 28-2-2027.
The medical certificate (class 1) was valid until 1-7-2025.
Flying experience
|
|
Latest 24 hours |
Latest 90 days |
Total |
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All types |
4 |
79 |
9,145 |
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This type |
4 |
79 |
5,500 |
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Landings this type |
15 |
47 |
5,000 |
Aircraft information
General information
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Manufacturer: |
De Havilland Aircraft of Canada (DHC) |
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Type: |
DHC-6 Series 300 (Twin Otter) |
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Serial number: |
350 |
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Airworthiness review certificate: |
Valid until 28-2-2026 |
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Engine manufacturer: |
2 x Pratt & Whitney Canada |
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Engine type: |
PT6A-27 |
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LH Engine Time Since New (TSN): |
28,943.4 hrs |
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LH Engine Cycles Since New (CSN): |
29,382 |
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LH Engine Time Since Overhaul (TSO): |
405.7 hrs |
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RH Engine TSN: |
12,935.7 hrs |
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RH Engine CSN: |
12,844 |
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RH Engine TSO: |
132.4 hrs |
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Propeller manufacturer: |
Hartzell HC-B3TN-3D |
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Propeller type: |
Constant speed |
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Maximum take-off mass (MTOM): |
12,500 lbs |
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Maximum landing mass (MLM): |
12,300 lbs |
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Fuel on board at the time of the serious incident: |
2,200 lbs |
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Aircraft total flight hours: |
35,831.0 hrs |
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Aircraft total flight cycles: |
63,482 |
Engine general
The PT6A-27 was a free-turbine turboprop engine. It consisted of a gas generator section and a turbine power section.
The gas generator section consisted of three axial compressor stages and one centrifugal compressor stage, a reverse flow annular combustor and a single stage turbine which drove the compressors and accessory gearbox. The power turbine was a single stage turbine which drove the propeller through a reduction gearbox.
Engine Fuel Control System
Each engine fuel control system consisted of an engine-driven HP fuel pump and a FCU. The HP fuel pump was installed on and driven through the accessory gearbox at the rear of the engine. The FCU was installed on the HP fuel pump and driven through HP fuel pump.
Electrical boost pumps in the forward and aft fuselage tanks supplied fuel to the inlet port of the HP fuel pump. The HP fuel pump increased the supplied fuel pressure to the FCU.
The FCU was a mechanical hydro pneumatic type fuel control.
The FCU metered the fuel according to the throttle lever position in the cockpit using the engine compressor third stage discharged air (P3) and a governor (GOV) within the FCU. The metered fuel was supplied to the fuel nozzles in the combustion chamber.
Fuel levers in the cockpit were connected to fuel cutoff valves.
Control of the engine fuel control system was mechanically by pulleys and cable systems which connected the throttle lever and fuel lever to the FCU and fuel cutoff valve.
Fuel tanks
Fuel was contained in forward and aft fuselage tanks located in the lower fuselage underneath the cabin floor and in smaller fuel tanks in each outer wing.
A filler neck and cap for each tank were located on the left side of the fuselage. Each of the fuselage tanks consisted of four interconnected cells, of which one cell was a collector cell. Each collector cell had two booster pumps to supply the engines.
The forward tank supplied the right engine and the aft tank supplied the left engine. There was a crossfeed valve between the forward tank and aft tank.
Each fuselage tank could be drained through a water drain at the collector tank. Each wing tank could be drained through a water drain at the lower area of the wing tank.
Fuel filters were installed in the fuel lines to each engine.
On the ground only, the lower fuselage fuel tanks could be used to fill the wing tanks. The wing tanks also had overwing filler caps.
The fuel in the wing tanks could not be transferred to the lower fuselage fuel tanks.
Engine ignition system
Two igniter plugs were installed in the combustion chamber of each engine. The two igniter plugs received their electrical power from an ignition exciter unit, which was supplied by the auxiliary battery bus.
The ignition was controlled by an ignition switch in the cockpit. The ignition switch had two positions: NORMAL and MANUAL.
When the ignition switch was set to NORMAL, electrical power was only provided to the ignition circuit during engine start (controlled through the respective engine start switch position).
When the ignition switch was set to MANUAL, the ignition system provided continuous electrical power to the igniter plugs of both engines simultaneously. MANUAL selection was for flight during severe turbulence, severe rain or icing conditions only. During all other phases of flight, the ignition switch was set to NORMAL, meaning the ignition circuit was de-energized. This was reflected in the POH and Aircraft Flight Manual (AFM) normal procedure checklist.
Approved fuels
Fuel type Kerosene Grades Jet A, Jet A-1 and F-34 (NATO code for JP-8).
Several other fuels were approved, the above list is not complete.
Meteorological information
Terminal Aerodrome Forecast (TAF)
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TAF EKYT |
291703Z 2918/3018 29010KT 9999 FEW120 BECMG 2922/2924 2500 BR BCFG BKN003 TEMPO 3000/3007 4000 BR BKN006 BECMG 3007/3009 9999 NSW SCT020= |
Aviation Routine Weather Weather Report (METAR)
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METAR EKYT |
291850Z AUTO 27011KT 9999 BKN140/// 12/08 Q1023= |
Aerodrome information
General information
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Aerodrome Reference Point: |
57 05 34N 09 50 57E |
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Elevation: |
8 ft |
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Runway directions: |
08L/26R and 08R/26L |
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Runway dimensions 26R: |
2,650 meters (m) x 45 m |
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Runway surface: |
Asphalt |
Flight recorders
The aircraft was equipped with a Garmin G950 Electronic Flight Instrument System (EFIS) which recorded engine, aircraft and flight parameters.
The data was of good quality and useful for the investigation.
The operator downloaded the recorded data and shared an electronic copy with the AIB.
Further the operator supplied an illustration of the plotted engine parameters during the serious incident, see Appendix 1.
Operator technical safety investigation
Prior to contacting the Danish AIB the operator performed the technical inspections and tests listed below:
- No fuel leaks were observed from the aircraft.
- Fuel samples were taken from all the fuel tanks and sent for analysis.
- Fuel samples were checked for water contaminations without remarks.
- Fuel tank vent system was checked without remarks.
- LH & RH engine HP fuel pump inlet and outlet filters were checked without remarks.
- Fuel system filters were checked without remarks.
- All the fuel tanks were defueled/emptied of F-34 fuel.
- All the fuel tanks were refilled with Jet A-1 fuel.
- All the air connections to the FCU were checked for obstructions/contaminations without remarks.
- Rigging of the throttle levers to the FCUs were checked without remarks.
- LH & RH engine first stage compressor blades were checked without remarks.
- Engine deacceleration and acceleration attempts with the engines running simultaneously were carried out, and the RH engine flamed out twice during deceleration. The LH engine did not flame out.
- The engine bleed valves were swapped.
- Engine deceleration and acceleration attempts with the both engines running simultaneously was carried out, and the RH engine flamed out during the third attempt. The LH engine did not flame out.
- The RH FCU and HP fuel pump were replaced.
- Hereafter several engine deceleration and acceleration attempts were carried out with no flame outs.
- The operator decided also to replace the LH FCU and HP fuel pump due to the initial engine flame out of the LH engine.
- During removal of the LH & RH FCUs and HP fuel pumps from the engines nothing abnormal was found.
Data recorded prior to the serious incident indicated that both the LH & RH engine showed fuel flows at idle and takeoff power to be within the specified ranges according to the aircraft manufacturer.
Data recorded showed that the engine gas generator speed (Ng) idle speed of the LH & RH engine prior to the serious incident were within specified range of 51-52%.
Data recorded showed that the fuel flows at idle and at takeoff power prior to and after replacement of the LH & RH FCU and HP fuel pumps, were comparable.
Fuel samples from the aircraft fuel tanks, the military fuel trucks and stationary fuel tanks, which were used to refuel the aircraft earlier the same day were sent for hydrocarbon pattern comparison checks.
The results of these tests indicated that the hydrocarbon pattern of all the fuel samples were the same, which concluded that the used fuel had not been contaminated with other fuel types.
In addition, fuel samples from the aircraft fuel tanks were sent to an external accredited laboratory, which confirmed that the fuel complied with the specific fuel specification.
AIB technical safety investigation
The AIB shipped the removed LH & RH FCUs and HP fuel pumps to the engine manufacturer for further tests and tear down inspections.
An accredited representative from TSB Canada participated in the tests and tear down inspections at the engine manufacturer.
All the four components had a TSO/ Cycles Since Overhaul (CSO) equal to 132.4 hrs/ 97.
Engine manufacturer examinations:
All four components were tested individually in test benches.
LH & RH FCUs:
- External inspection showed the units were in a condition typical for units returning from service.
- On the LH FCU no fuel inlet strainer was installed.
- On the RH FCU a clean inlet strainer was installed.
- Simulation of the serious incident with the FCUs installed, as received, in the test bench was performed. Throttle lever was rotated to maximum position and then back to idle position within 1 second 10 times with the FCUs responding satisfactorily to the throttle inputs.
- With the throttle lever in idle the fuel flows did not reduce below the minimum fuel flow level.
- The FCUs were tested according to the Original Equipment Manufacturer (OEM) Component Maintenance Manual (CMM). Tests showed that the FCU subsystems were functional with most deviations related to allowable field adjustments.
- On the LH FCU the minimum and maximum fuel flows were outside the calibration limits but were not considered a potential contributor to the serious incident.
- On the RH FCU the maximum fuel flow was outside the calibration limit but was not considered a potential contributor to the serious incident.
- Teardown inspections of the FCUs gave no rise to remarks, which could explain the serious incident.
- No evidence of fuel contamination was visually detected in the fuel side circuit of the FCUs.
LH & RH HP Fuel Pumps:
- External inspection showed the units were in a condition typical for units returning from service.
- HP fuel pumps outlet filters were clean.
- LH HP fuel pump inlet strainer was clean.
- RH HP fuel pump inlet strainer showed presence of some particulate debris but was not considered a potential contributor to the serious incident.
- The HP fuel pumps were tested according to the OEM CMM, which gave no rise to remarks.
- Teardown inspections of the HP fuel pumps gave no rise to remarks.
Additional information
Fuel used and uploaded
Earlier the same day, the aircraft had been fuelled with F-34 from a military fuel truck at EKYT.
F-34 fuel is military kerosene aviation fuel similar to commercial kerosene aviation fuel Jet A-1 with the following additives added:
- lubricity improver additive
- Fuel system icing inhibitor additives.
Operators use of Jet A-1
Prior to and during the serious incident the operator used F-34 fuel.
For a period of time initiated after replacement of the FCUs and HP fuel pumps the operator only operated the aircraft using Jet A-1. Hereafter F-34 was used again.
Operational status from the operator
Since the replacement of the FCUs and HP fuel pumps in May 2025 the operator had not experienced any engine flameouts.
Engine In Flight Shut Downs (IFSD)
The engine manufacturer informed the AIB that engine In Flight Shut Downs (IFSD) rate, which amongst others included engine flameouts, was 3 x 10-6 flight hours.
Continuous ignition
Continuous ignition keeps the igniters operating continuously in the turbine engine’s combustion chamber, providing a constant source of ignition so that the combustion is maintained or immediately re-established if the flame is temporarily lost.
Some aircraft and engine manufacturers use it typically during takeoff and landing, heavy rain or hail, moderate to severe turbulence, icing conditions and operations from contaminated runways.
The use of continuous ignition depends on the engine design and the engine’s susceptibility to water, ice or airflow disturbances.
Analysis
General
The instructor was properly licensed.
The aircraft was airworthy.
The weather was not considered a contributing factor to the serious incident.
Technical safety investigation
The illustration shown in Appendix 1 showed, that at time 18:51:30 + 35 seconds, the LH engine speed (E1Ng (%)) decreased and the engine flamed out. The RH engine speed (E2Ng (%)) increased followed by a decrease and a flameout.
There was sufficient fuel onboard the aircraft.
The fuel type used was approved for use on the engines.
The fuel fulfilled the specification and was not contaminated.
The fuel levers, fuel shut off valves and fuel pumps were checked set in the correct positions shortly after the serious incident.
Data recorded prior to the serious incident indicated that both the LH & RH engine showed fuel flows at idle and takeoff power to be within the specified ranges according to the aircraft manufacturer.
Data recorded showed that the engine gas generator speed (Ng) idle speed of the LH & RH engine prior to the serious incident were within specified range of 51-52%.
The technical inspection of the fuel filters, air lines to the FCUs and rigging gave no rise to remarks.
Subsequent engine acceleration and deceleration test runs with both engines running resulted in some flameouts on the RH engine only.
After replacing the RH HP fuel pump and FCU, and carrying out subsequent engine acceleration and deceleration test runs with both engines running, no flameouts were observed.
This resulted in the need for additional testing and inspections of the RH HP fuel pump and FCU.
The LH engine flameout only happened once, which was during the serious incident.
The operator decided also to replace the LH HP fuel pump and FCU.
Further bench tests at the engine manufacturer, which included simulation of acceleration and deceleration tests and tear down inspection of the LH & RH HP fuel pumps and FCUs did not reveal anything, which could explain the engine flameouts.
Data recorded showed that the fuel flows at idle and at takeoff power prior to and after replacement of the LH & RH FCUs and HP fuel pumps, were comparable.
The technical inspections of the aircraft and the individual inspections and tests of the FCUs and HP fuel pumps indicated that fuel starvation was unlikely.
The operator has not, since replacement of the HP fuel pumps and the FCUs in May 2025, experienced any flameouts during operation of the aircraft.
Operational safety investigation
The ignition was set to NORMAL position during the serious incident.
It was not a procedural requirement in the aircraft manufacturer’s POH and AFM to have ignition set to MANUAL position during approach, landing and takeoff to reduce the risk of engine flameout during critical flight phases.
It was not possible to determine the percentage of IFSD that were due to engine flameouts.
However, the recordings, and the technical inspections and tests indicated, that the engines received the correct amount of fuel and air during the serious incident.
It therefore seems unlikely that the engines would have flamed out, if the ignition had been activated.
Conclusions
During a takeoff ground roll when the student pilot applied full power, after the power had been in idle, the LH engine had flamed out. The instructor pulled back both throttle levers to idle and subsequently the RH engine flamed out.
Aircraft inspections and engine ground run tests indicated that the reason for the engine flameouts might be located to the FCUs and/or the HP fuel pumps.
The engine manufacturer performed bench testing and tear down inspections of the FCUs and the HP fuel pumps without any findings, which could explain the engine flameouts.
The AIB safety investigation was unable to determine the cause of the engine flameouts.
Continuous ignition is applied on some aircraft types during critical phases of flight as a general mitigation against flameout risk, however, no causal relationship has been identified in this occurrence, and no safety recommendations are issued.
Appendices
Appendix 1
Contact the Accident Investigation Board
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