Wideroe DH8A near Sorkjosen on Feb 23rd 2006, both propellers oversped, serious engine damage

Last Update: June 26, 2012 / 14:46:31 GMT/Zulu time

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Incident Facts

Date of incident
Feb 23, 2006

Classification
Report

Airline
Wideroe Airlines

Flight number
WF-922

Departure
Tromso, Norway

Destination
Sorkjosen, Norway

Aircraft Registration
LN-WIE

Aircraft Type
De Havilland Dash 8 (100)

ICAO Type Designator
DH8A

A Wideroe de Havilland Dash 8-100, registration LN-WIE performing flight WF-922 from Tromso to Sorkjosen (Norway) with 17 passengers and 3 crew, was in the initial descent towards Sorkjosen when the aircraft encountered severe turbulence. The captain (51, ATPL, 11,800 hours total, 4,410 hours on type), familiar with the terrain and the turbulence scenario, gripped both power levers to pull them back to flight idle and thus reduce speed of 225 KIAS to the maximum speed in turbulence of 180 KIAS, however, the aircraft began to severely roll right and the nose pitched down, dust in the cockpit was thrown up, a smell of oil developed and a deafening noise came from the propellers disabling all cockpit communication and a large number of warning lights illuminated. The aircraft descended about 1000 feet out of control until the captain was able to regain control and stabilize the aircraft at a heading of 090 instead of original heading of 060. However, the speed continued to reduce despite full power, the deafening sound reduced however enabling communication again. The captain recognized the right hand propeller was rotating at too high a speed, called "propeller overspeed" prompting the first officer (29, CPL, 1,680 hours total, 126 hours on type), pilot monitoring, to verify it was the right engine in overspeed and action the relevant memory checklist shutting the right hand engine (PW121) down, however, the propeller did not feather at the first attempt and went into feather after selecting alternate feather 214 seconds after the propeller oversped. The aircraft began to immediately climb again after the right hand propeller had reached its feathered position. The crew decided to return to Tromso where the aircraft landed safely on single engine. No injuries occurred, the right hand engine received substantial damage.

Two days later maintenance determined both propellers had oversped and reported the occurrence to the Norwegian Statens Havarikommisjon (also known as Accident Investigation Board Norway AIBN).

Six years later the AIBN released their final report concluding the probable cause of the serious incident was:

In an attempt to reduce the aircraft's speed during increasing turbulence, the Commander inadvertently pulled both Power Levers past Flight Idle. This was not prevented by the built-in protection systems, and both propellers reached uncontrollably high rotation speeds. The right engine was severely damaged and control of the aircraft was partly lost. As the situation was brought under control, the crew managed to return and land safely in Tromso with one engine in operation. Widerøe has modified its aircraft to prevent recurrences.

The AIBN reported that the flight data recorder showed vertical acceleration reached -1.07G briefly during the encounter, otherwise varying between +0.2G and +2.0G. The aircraft reached a maximum bank angle of 58.4 degrees to the right and a pitch attitude of 19.9 degrees nose down, the speed of the right hand propeller accelerated from 911 rpm to more than 1500 rpm (maximum recordable speed 1500 rpm) and the left hand propeller accelerated from 916 rpm to 1483 rpm within a period of 7 seconds, during those 7 seconds the engine torque reduced from 55% to 0%. The upset began at an altitude of 8870 feet MSL, the aircraft lost 760 feet during the upset. The aircraft was doing 225 KIAS and accelerated to 243 KIAS within 10 seconds prior to propeller acceleration. 2 seconds after the left hand propeller reached 1483 rpm the propeller slowed again and returned to 914rpm, the right hand engine remained at 1500+ rpm for about 17 seconds and did not return to a normal rotating speed until being feathered 214 seconds after the onset.

Following landing it was discovered that the right hand engine had received substantial damage, for example the shaft between engine and gear box had separated and thrown fragments through the shaft housing and into the air intake.

Following the discovery both engines had oversped both engines and propellers were removed from the aircraft. The engines were sent to the engine manufacturer for further examination, the left hand propeller was examined by Wideroe's technical department and the right hand propeller was scrapped without further examination due to the loads received in flight. The engine manufacturer found the left hand engine had received no damage, the left hand propeller was also found fault free by Wideroe. In addition, both PCUs and OSGs were examined by the manufacturer and found fault free.

The engine manufacturer found the right hand engine had received following damage:
- shaft between engine and gear box had separated
- low pressure turbine shaft had separated and partially melted
- high pressure turbine shaft had separated and partially melted
- major damage to the vane rings of the second stage power turbine
- most of the second stage power turbine blades had been completely destroyed
- considerable damage between rotational and stationary parts of the engine
- significant damage to the compressor due to metal fragments passing through it

The AIBN reported that each propeller is governed by a propeller control unit (PCU), which uses hydraulic oil pressure to adjust the blade angle to maintain a rotational speed up to 1212 rpm selected by the pilots. In addition an overspeed governer (OSG) will increase the blade angle if a propeller overspeed (rpm greater than 1236 rpm) is detected, if this is not sufficient and the propeller speeds above 1308 rpm the OSG will also pneumatically reduce the engine's fuel supply. During ground operation however the PCU however does not control the propeller speed which then is the result of selected blade angle and fuel supplied to the engine, the so called "Beta Range". If the blade angle becomes negative the propeller receives energy from the oncoming airflow and thus increases rotational speed, if the propeller is adjusted at maximum angle with its leading edge straight into the oncoming airflow (feathered position) no energy is received from the airflow and little drag is being generated.

The power levers control the power generated by the turbine in all range of movement. To enter the power range for ground movement (beta range) however the power lever needs to be lifted and moved aft of the flight idle gate. As such, any movement of the power levers aft of flight idle while airborne creates the risk of the propellers overspeeding and a dramatic increase of drag to the point of losing control of the aircraft. Therefore, in order to prevent unintentional activation of the beta range, each power lever has protection and warning mechanisms, a small release trigger must be lifted (required power about 1.4kg/13.7N) by about 5mm in order to disengage a mechanical stop. In addition an intense warning sound will be triggered while airborne.

The AIBN analysed that there are similiarities to the crash of P2-MCJ, see Crash: PNG DH8A near Madang on Oct 13th 2011, both propellers oversped and also mentioned their own earlier investigation into the accident of LN-WIK at Hammerfest, see Report: Wideroe DH8A on May 1st 2005, gear collapse on landing stating that the crew had heard sounds like bird twittering in that encounter which later turned out to be the warning sound for pulling the power levers into the beta range while airborne. The AIBN referenced further occurrences with propellers inadvertently entering beta range on Fokker 50s, Embraer EMB-120s and other Dash 8s.

The AIBN reported that following first findings an interim safety recommendation was released by the AIBN to Bombardier, who had taken de Havilland over, and the Canadian Transportation Safety Board in February 2007 reading:

"The AIBN recommends that Bombardier evaluate all DHC-8 models with respect to inadvertent airborne reversing. All models that can be reversed unintentionally during pull back of Power Levers should be modified in such a manner that dangerous inadvertent airborne reversing is unlikely to happen. Until a modification is implemented operators should be informed about the hazard in an appropriate way. (Interim safety recommendation no. 06/120-9)"

This safety recommendation however was met with rejection by both Bombardier and Canada's TSB in May 2007 (editorial note: consider this statement in the light of the events on P2-MCJ in 2011, four years later, as documented so far by the preliminary report):

"Bombardier appreciates the opportunity to review and comment on the above referenced safety recommendation. We have thoroughly reviewed the existing power lever flight idle gate design and find that inadvertent airborne reversing is unlikely to occur. In our opinion, further modification to the installation is not necessary."

Wideroe took an immediate safety action by modifying the mechanical stop mechanism in accordance with a Bombardier Safety Bulletin, which necessitates the power lever to be pulled back to flight idle before the mechanical lock release trigger can be lifted.

The AIBN analysed that had power been lost from both engines as result of the power levers being pulled back aft of the flight idle gate, a successful emergency landing would have been unlikely given darkness, altitude and distance to Sorkjosen aerodrome. It was pure chance that the left hand engine did not reach the same degrees of propeller overspeed as the right hand engine did and thus escaped receiving similiar damage to the right hand engine.

The AIBN analysed further: "The incident was not caused by shortcomings in operational procedures, but by the Power Levers unintentionally pulled back so that the propellers entered Beta-Range. That this happened in spite the crew being familiar with the hazard shows that the protection and warning functions were insufficient. There is nothing to indicate that technical malfunctions caused the incident."

The AIBN went on to say that the release triggers are lifted routinely in connection with landings and the action can not be considered unusual or exceptional.

In addition "it requires more precision to grip the Power Levers without the fingers closing around the handles than it does to grip the entire mechanism. This factor is especially relevant when the aircraft is subjected to turbulence. The Accident Investigation Board can therefore understand how the Commander came to grip the entire mechanism, including the release triggers, when he suddenly had to pull back the Power Levers to Flight Idle. In this period, the g values were as low as - 1.077. ... It is therefore understandable that the release triggers may have been lifted by accident when the Commander pulled the Power Levers to Flight Idle. A force of 2 kg, which is required to pass Flight Idle, cannot be said to represent certain assurance against the Power Levers being pulled too far back. This is especially true when the aircraft is being shaken hard in turbulence."

The AIBN analysed that the safety action taken by Wideroe significantly reduced the possibility of a recurrence.
Aircraft Registration Data
Registration mark
LN-WIE
Country of Registration
Norway
Date of Registration
AigdghmgnAcjld j Subscribe to unlock
Manufacturer
DE HAVILLAND AIRCRAFT OF CANADA LIMITED
Aircraft Model / Type
DHC-8-103
ICAO Aircraft Type
DH8A
Serial Number
Bqb Subscribe to unlock
Maximum Take off Mass (MTOM) [kg]
KqmAk Subscribe to unlock
Engine Count
H Subscribe to unlock
Main Owner
BilAcckjmfhlqignbnc gjpcjfAneepgAbqebphmfAmmhenjd Subscribe to unlock
Incident Facts

Date of incident
Feb 23, 2006

Classification
Report

Airline
Wideroe Airlines

Flight number
WF-922

Departure
Tromso, Norway

Destination
Sorkjosen, Norway

Aircraft Registration
LN-WIE

Aircraft Type
De Havilland Dash 8 (100)

ICAO Type Designator
DH8A

This article is published under license from Avherald.com. © of text by Avherald.com.
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