Propeller blade collar failure involving de Havilland DHC-8, VH-ZZA, near Darwin, Northern Territory, on 3 December 2019

The occurrence

On 3 December 2019, a de Havilland Canada DHC-8-202 (Dash 8) aircraft, registered VH‑ZZA (ZZA) and operated by Cobham Aviation Services Australia (Cobham), departed Darwin Airport, Northern Territory shortly before 1200 Central Standard Time^[1] to conduct aerial work. At 1200, on climb between 1,000 and 2,000 ft above mean sea level, the flight crew heard a loud bang. One crewmember saw something fly past the window, and assumed a birdstrike had occurred. There were no issues with controllability and all systems were functioning normally, so the crew elected to continue with the mission.

The aircraft landed without further incident, but an engineering inspection subsequently found damage to the right propeller and to the ice shield on the right side of the fuselage. Subsequent removal of the ice shield revealed that the fuselage had been penetrated in two places (Figure 1).

Figure 1: Fuselage penetration

Source: Cobham

The right propeller showed signs of damage on the leading edge of the number 1 blade, and the number 2 blade collar was missing. Cobham’s engineering team reported that the collar had separated from the number 2 blade and struck the number 1 blade. No pieces of the collar were found and there were no other components missing from the aircraft that could have damaged the fuselage.

Context

Blade collar design

The blade collar was a plastic component fixed to each propeller blade to improve aerodynamic performance. The two halves of the collar, such as those shown in Figure 2, were screwed together and secured to the blade shank by an adhesive. The two approved adhesives for installation were RTV157 and PR-1826 Class B.

Figure 2: Blade collar halves

Source: Cobham Aviation Service Australia

In 2015, the blade manufacturer, Collins Aerospace, released an updated design for the blade collar as the original design was not dimensionally stable, which prevented consistent bonding with the blade. This in turn caused a number of collars to separate from the blade shank during service. The new design was only to be installed using RTV157 adhesive.

To reduce the potential loss of adhesion, Collins Aerospace also released Action Item D9274‑AI07499 on October 3, 2019. This item added the use of primer SS4004P during the collar bonding operation.

Propeller maintenance history

An overhauled Hamilton Sundstrand 14SF propeller was fitted to the right engine of ZZA in 2016. The blade collars were the original design, as the updated collars were unavailable. On 2 March 2018, the number 2 and number 3 blade collars were found to be loose. Cobham’s engineering team inspected and refitted both collars in a field repair, as the updated blade collars had limited availability.

Nine days later, on 11 March 2018, the number 2 blade collar was found to have moved around the blade, indicating it had de-bonded. Cobham subsequently replaced it with the updated collar design in another field repair. At this time, the use of primer had not yet been included in the component maintenance manual. There was no further maintenance performed on either the number 2 or number 3 blades, and prior to the incident they were last inspected on 15 October 2019.

Post-occurrence inspection

Following the collar separation, the number 2 blade was sent to Cobham’s propeller repair vendor for examination. The number 3 blade was also sent for examination because it had last been repaired at the same time as the number 2 blade.

While the number 2 blade collar was never recovered, the impression left in the remaining adhesive indicated that the collar had been abraded prior to installation, as required. The adhesive remaining on the blade was RTV157, the correct type for the updated collar design. The vendor also noted, however that:

The remaining adhesive was easily peeled from the blade shank, indicating poor adhesion.

The vendor determined that this was most likely due to insufficient surface preparation/cleaning prior to adhesion. Figure 3 shows the adhesive being peeled off the number 2 blade shank where the separated collar was previously attached.

Figure 3: RTV157 adhesive being peeled off the number 2 blade shank

Source: Propeller repair vendor

The number 3 blade collar was the original design, so both adhesives were permitted. The rear half was securely bonded to the blade with PR-1826, however the front half had been re-attached with RTV157. PR-1826 still coated the blade shank and the vendor reported no adhesion between the different adhesives. As a result, the front collar half was loose.

The component maintenance manual required old adhesive to be removed from the blade shank before a collar was re-attached.

Similar occurrences

A search of the ATSB’s database found similar occurrences involving blade collar separation from the same propeller type fitted to Dash 8 aircraft:

ATSB investigation 200304918

In 2003, a Dash 8 experienced an in-flight blade collar separation shortly after take-off from Sydney, New South Wales. The subsequent examination revealed that the adhesive used had been contaminated. The source of the contamination could not be identified. The same operator experienced another collar separation in 2004. Subsequent inspections on the rest of the fleet indicated five other aircraft with at least one loose blade collar. As a result, the operator initiated a repetitive collar inspection regime on their fleet.

ATSB occurrence 201203474

In 2012, on final approach into Port Macquarie Airport, New South Wales, the flight crew of a Dash 8 heard an impact noise on the left side of the aircraft. Engineering later determined that a blade collar had separated from the left propeller before striking a blade and then the engine nacelle. No potential reason for collar separation was provided to the ATSB.

Internationally, de Havilland Canada identified one occurrence in which the fuselage of a DHC-8 aircraft may have been penetrated by collar fragments. In October 2011, a post-flight inspection of a DHC-8-314 revealed part of a blade cuff missing, along with damage to one of the propeller blades and two holes in the fuselage. While no debris was found, it was concluded that fragments of the collar or attaching hardware likely struck the blade and was propelled into the fuselage.

Analysis

The adhesive remaining on the number 2 blade was not well bonded to the blade shank, likely due to problems with the preparation/cleaning of the bonding surfaces prior to adhesion. The lack of adhesion between the collar and blade shank resulted in the collar becoming loose and separating in flight. Similar problems were observed on the number 3 blade, where adhesive from a previous field repair prevented adequate bonding between the blade and collar.

The number 2 blade collar was the only component found missing from the aircraft during the post‑flight inspection. Given that the number 1 blade was also damaged, it follows that the number 1 blade accelerated the detached collar fragments into the fuselage with sufficient force to penetrate it. This appears to be consistent with a previous occurrence, where it was determined that fragments of blade collar likely struck a propeller blade, accelerating them into the fuselage.

This incident illustrates that component failure and in‑flight damage may not always be apparent to flight crew, and there are potential risks in incorrectly attributing the cause of events such as a loud bang. As such, serious consideration should be given to terminating the flight following any unexplained abnormal indication, especially if it occurs in proximity to a suitable airport.

Findings

From the evidence available, the following findings are made with respect to the propeller blade collar failure of a de Havilland DHC-8-202, registered VH-ZZA, that occurred near Darwin, Northern Territory, on 3 December 2019.

Contributing factors

When the number 2 blade collar was replaced, there was a lack of adhesion between the collar and blade, likely due to improper preparation of the bonding surfaces. This resulted in the collar debonding and separating in flight.

• The detached number 2 propeller blade collar struck the number 1 propeller blade. This accelerated collar fragments into the aircraft's fuselage with sufficient force to penetrate it.

Safety issues and actions

Safety action by Cobham Aviation Services Australia

As a result of this occurrence, Cobham released an engineering notice in March 2020 requiring the entire blade assembly to be replaced in the event of a loose or cracked blade collar. If a serviceable blade assembly was not available, Cobham required collars to be replaced in consultation with the relevant Technical Services Engineer and emphasised that all preparation instructions from the component maintenance manual were to be observed during the repair.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• Cobham Aviation Services Australia
• the propeller repair vendor
• Collins Aerospace
• De Havilland Canada.

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report.

A draft of this report was provided to the following directly involved parties:

• the flight crew
• Cobham Aviation Services Australia
• De Havilland Canada
• Collins Aerospace
• the propeller repair vendor
• the Civil Aviation Safety Authority
• the Transportation Safety Board of Canada.

Submissions were received from:

• the flight crew
• Cobham Aviation Services Australia
• Collins Aerospace.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

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1. Central Standard Time (CST): Coordinated Universal Time (UTC) +9.5 hours