Double engine failure

Citation

(2002), "Double engine failure", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 3. https://doi.org/10.1108/aeat.2002.12774cab.015

Double engine failure

Keywords: Aircraft, Safety, Engine

On a DHC-8-311 aircraft the commander was a company line training captain who was carrying out linc training for a new first officer. The first two sectors of the day's duty had been uneventful and the aircraft landed at Bristol at 1314hrs. At that time, the weather produced a surface wind from 080 at 25 gusting 36 knots, visibility 2,000 metres in light snow, scattered cloud base 500 feet, temperature 0°C. The aircraft parked normally on stand 5, heading east (into wind). After the passengers had deplaned, the crew went into the airport terminal, as the next departure was at 1510 hrs. The crew did not fit the engine intake blanking plugs during the turnround.

During the intervening period, the weather conditions worsened. The snow fall became heavier with a progressive deterioration in visibility and the strong, gusty easterly wind continued. During its time on the ground, the aircraft began to accumulate a covering of snow. The crew returned to the aircraft in time to prepare for the next planned departure. The commander arranged for the aircraft to be deiced using heated type II fluid. This was carried out and the commander completed a pre-flight external inspection which included a visual inspection from the ground of each engine intake lip and a tactile inspection of the rear of each intake through the open bypass doors. The conmmander assessed that there was no build up of snow or slush in these areas and considered that the engine intakes were clear of ice and snow.

The runway in use was Runway 09 which had been closed for a period of time to enable snow clearance operations to take place. Meanwhile, the passengers had been embarked. Once the runway had been re-opened, the crew performed a normal engine start and the the aircraft began to taxi out for departure at 1535 hrs. On reaching the holding point for Runway 09, ATC requested that the aircraft hold position in order to allow a stream of inbound aircraft to land. While holding, with the aircraft's tail into wind, the right engine suddenly ran down, for no apparent reason. The crew carried out the Engine Failure procedure from the aircraft's Quick Reference Handbook (QRH), but did not select Ignition to Manual for the left engine. About 2.25 minutes after the right engine failure had occurred, the left engine also failed without warning. The weather at this time was a surface wind from 080 at 24 knots, visibility 1,000 metres in moderate snow, scattered cloud base 300 feet, temperature 0°C.

The aircraft was thus left on the runway with only battery power. The commander advised ATC of the occurrence and requested provision of a bus to deplane the passengers and a tug and towbar to reposition the aircraft to the parking ramp.

The passengers were duly deplaned, having been carefully advised by the cabin crew to avoid the left propeller which was still windmilling quite fast adjacent to the forward exit door. The aircraft was then towed to a parking stand. On arrival the aircraft was quarantined and the engine intake blanks were fitted. Fortunately, the outside air temperature remained at freezing until the AAIB inspection of the aircraft was carried out.

Details and engineering inspection

On the engines, air is fed to the centrifugal first stage of the compressor by a chin- mounted intake (see Figure 1). The lip of the intake has a rubber de-ice "boot" and, as the air flow turns upward towards the compressor inlet, it passes through a heated seal forming pan of the engine itself. In icing or heavy precipitation conditions, selection of the bypass door to 'open' means that in forward flight, some of the ram-air and particles of ice or heavy droplets of water should pass by the compressor inlet, into the plenum and out through the bypass aperture.

Figure 1 Drawing of engine and intake showing by-pass door open at 'A'

The aircraft was examined on the ramp at Bristol Airport. As found, both engines had intake blanks fitted, the intake bypass doors were open and both propellers were unfeathered. The wet snow had recently ceased and the temperature was between 0 and +1°C. An initial "feel" by hand inside the bypass door exit duct suggested that there was no snow/ice present in the duct but when the blanks were removed and a stepladder and torch obtained, it was possible to discern accumulations of wet slush behind the intake lip and further down in the bypass plenum (see Plates 10 and 11). These deposits were similar in both engine nacelles and, in the ease of the plenum, were about 25-40 mm deep, mainly around the edges of the cavity and forward of the bypass door (see Plate 10). It was for this reason that they had not been detected by the tactile check earlier. The snow/slush was removed, the engines were started and the aircraft taxied to a ground-running apron, where the engines and propellers were exercised throughout their operating range without any anomalies being discovered. The engines were further inspected before the aircraft was returned to service.

Plate 10 View looking aft into intake showing residual slush deposits

Plate 11 View looking into plenum with by-pass exit just visible

In the absence of any defects or malfunctions associated with the engines it is concluded that they both probably 'flamed- out' due to sudden ingestion of wet snow/ slush which had accumulated in the intake and/or bypass plenum whilst the aircraft was parked. The deposits which were observed were probably the residue of larger accumulations which had been ingested during the incident.

Previous occurrences

The phenomenon of in-flight, ice induced engine flameouts had been experienced on previous occasions on other DHC-8 aircraft belonging to overseas operators. In response to these events, the manufacturer issued a Safety of Flight Supplement, number 4, on 28 July 1994. In each occurrence the aircraft had been parked in heavy precipitation in temperatures near freezing without having the engine intake plugs fitted. Investigation of these occurrences concluded that quantities of ice/slush/snow had been deposited within the engine intake nacelles. The sudden breaking loose of this build up could disturb the intake airflow sufficiently to cause an engine flameout. The Suppicment stressed the importance of an engine intake inspection prior to flight. It also recommended that the first take-off after such exposure should be conducted with the Bypass Doors open and the ignition selected to Manual (on).

Operator's procedure

The manufacturer's Safety of Flight Supplement had been encompassed into this operator's procedure in the Operations Manual and in a Notice to Aircrew of 13 October 2000, entitled Winter Operations Dash 8. Page 4 of the Notice contained the following advice:

Through the icing experience of one operator it has been discovered that ice can build up in the engine air intake, immediately ahead of the bypass door. If this ice accumulation is not removed it can build forward of the nacelle plenum and potentially cause an engine power interruption. As a result of these events, (operator's name) has implemented the following procedure:

Tactile inspection of the engine intakes must be completed during all station stops when icing conditions exist. If icing conditions are encountered in-fight or icing conditions exist or have existed on the gound, an inspection to ensure that the air intakes are clear, must be performed. A visual inpection of the intake may NOT identify ice that has formed in the nacelle plenum. With the intake bypass doors "OPEN", reaching inside the plenum chamber will identify any ice, slush or "other" contaminant build-up. This area MUST be clear before flight.

The engine intakes of the DHC-8 are located below the propeller spinners. As such, they are significantly higher than even the tallest pilot. Thorough visual inspection of the inside of the intake without the aid of some form of elevated platform is not physically possible. In order to perform the visual check, it was usually necessary for the pilot to obtain some steps from a ground handling agency. These were not necessarily readily available.

As a consequence of this incident, the operator has equipped each aircraft in the fleet with its own set of foldaway steps to facilitate the intake inspections. These steps can be stowed in the baggage hold of each aircraft and are readily accessible to the pilots.

Because of the possibility of a repeat of this type of incident, AAIB proposed that the incorporation of Service Bulletin (SB) no 8-74-02, which linked igniter operation to engine HP spool speed (Nh), be mandated on all aircraft by the Transportation Safety Board of Canada (with this modification embodied, when the condition levers are selected above FUEL OFF, the igniters automatically operate if Nh falls below a threshold value). The Board however, responded negatively to this proposal, citing that the current maintenence and operational procedures afforded an equivalent level of protection to that offered by the SB. They considered that the choice of which of the two alternative ways of achieving the intent of the recommendation was a business decision which was best left to the operators. Within the UK, all operators of the Dash 8 have elected to modify their aircraft in accordance with the SB to afford maximum protection from this type of event. The manufacturer also indicated that it would revise and reissue DHC-8 Service Letters DH8-SL-12-006E (100/200/300 series aircraft) and DH8-12-001C (400 series) to highlight the importance of ensuring that the engine intakes are clear of frozen contaminant prior to departure.

ReferenceAAIB Bulletin 1/2002