[Peetwo]

Update: Botsy via the Oz -  

The Rex Saab and the missing propeller:-HERE.

Preliminary report published: 13 April 2017

The occurrence

On 17 March 2017, a Saab 340B aircraft, registered VH-NRX (NRX) was being operated as RXA768 on a routine passenger flight from Albury, New South Wales (NSW) to Sydney, NSW. On board the aircraft were 16 passengers and 3 crew.

About 55 nautical miles south-west of Sydney airport, the crew noticed uncommanded engine indications and began the necessary checklists. While undertaking the checklist items, the crew experienced minor vibrations from the right engine. These vibrations worsened as the checklist progressed and became visually evident to the First Officer.

As a result the crew commenced the engine shutdown procedure. During the engine shutdown procedure, the propeller separated from the aircraft. The crew made a Pan-Pan[1] call to air traffic control, and completed the engine shutdown procedure. The aircraft landed without incident at Sydney airport.

Figure 1: The aircraft, VH-NRX, at Sydney airport after the incident

Source: Grahame Hutchison

An inspection of the aircraft by the ATSB at Sydney airport identified that the propeller shaft had fractured, leading to the separation of the propeller.
On 21 March 2017, the NSW Police Aviation Support Branch (PolAir) undertook a search operation for the separated propeller. The propeller was located in an area under dense forest about 8NM south-west of Sydney airport.

Figure 2: The propeller that had separated from VH-NRX as found by PolAir about 8NM south-west of Sydney airport
Source: ATSB

The propeller was found with the flange section of propeller shaft secured to the propeller assembly and a fracture through the propeller shaft. ATSB subsequently removed the remaining propeller shaft and integral flange section (Figure 4) for examination at its facilities in Canberra.

Propeller shaft examination

The recovered part of the propeller shaft is highlighted in figure 3. The propeller was found properly secured to the forward-facing flange by bolts and the dowel pins pictured. The examination was conducted with representatives present from the Civil Aviation Safety Authority (CASA), SAAB, GE Aviation (engine manufacturer) and Regional Express (REX). Initial observations revealed cracking that appeared to run between the main shaft and the flange region. The part was sectioned in order to expose the crack’s fracture surface.

Figure 3: Propeller gearbox schematic highlighting the recovered section of the propeller shaft

Source: GE Aviation, modified by ATSB

The crack was found to be a fatigue fracture that had initiated within the propeller mounting flange, and then transitioned into the shaft section (see figure 4). The crack originated at the bore of a dowel pin near the forward face of the propeller hub flange. The dowel pin bore was corroded in parts (shown in figure 5), and corrosion pitting was found near the fracture. Further work is ongoing to ascertain whether the corrosion or other factors contributed to the fracture initiation.

Figure 4: Section of the propeller shaft showing the fatigue crack originating at the dowel hole and progressing into the shaft itself
Source: ATSB

This is the first known critical failure of this type initiating within the propeller hub flange of a GE Aviation CT7-9B engine. The same propeller gearbox (PGB) is fitted to multiple variants of the CT7 engine (5A2, 7A1, 9B, 9C, and 9C3) on SAAB 340 and EADS CASA[2] CN-235 aircraft. There is currently no maintenance requirements specified in existing maintenance manuals for routine inspection within the dowel pin bores. Any corrosion or cracking within the bore may go undetected until it progresses to the surface of the flange. Other than a visual inspection of the flange during propeller removal, inspection for surface defects (via magnetic particle inspection or dye penetrant inspection) only occurs when the PGB is disassembled for maintenance at a workshop specifically approved by the engine manufacturer.

Figure 5: Corrosion observed within the bore of the dowel pin hole

Source: ATSB

Safety advisory notice

AO-2017-032-SAN-001:

The ATSB advises that those responsible for the operation and maintenance of SAAB 340 and EADS CASA CN-235 aircraft fitted with the GE Aviation CT7 engine type variants 5A2, 7A1, 9B, 9C, and 9C3 should note the facts presented in this preliminary report with a view to addressing any risks to their own operation.

Proactive safety action taken by GE Aviation

GE Aviation is actively involved in supporting the Australian Transport Safety Bureau in this investigation. The propeller flange and all required hardware has been transported to GE Aviation laboratories in Cincinnati for further metallurgical analysis.

GE Aviation is inspecting additional PGBs from the fleet and recommends that all operators follow existing maintenance and inspection procedures. As the investigation progresses GE Aviation will release additional maintenance and inspection recommendations if they become necessary.

Proactive safety action taken by Regional Express

Regional Express has quarantined all propeller gearboxes with propeller shafts of the same series as that installed in VH-NRX.

Further investigation

The investigation is continuing and the ATSB will focus on:

• maintenance procedures associated with the PGB shaft
  
• factors that may have contributed to the fatigue fracture at the propeller mounting flange, possibly including:
  • design and manufacturing of the dowel pins, bores, and overall assembly
    
  • corrosion protection on the surface of the part
    
  • opportunities for crack detection.
    

Should any critical safety issues emerge during the course of the investigation, the ATSB will immediately bring those issues to the attention of the relevant authorities or organisation. This will allow those authorities and organisations to consider safety action to address the safety issues. Details of such safety issues and any safety action in response will be published on the ATSB website at www.atsb.gov.au . 

Speedy, sane and in all probability, the right answer - stressed component corrosion?. Well done ATSB, and GE.

In response to this half-assed attempt at aviation safety journalism...

Rex fractured propeller part not in maintenance program, ATSB report says
The Australian - ‎Apr 12, 2017‎
The fatigue fracture that caused a Rex plane to lose a propeller over southwest Sydney last month “may go undetected” because inspections of the part are not included in the regular maintenance program, Australia's safety authority has revealed.

...where Creepy (MKII) Clearly scapegoats Byron Bailey and spends half the article grovelling to John Sharp... :

..Last month, The Australian reported claims by veteran pilot Byron Bailey that had the pilots not taken action to shut down the engine, the propeller would have been spinning much faster and then impacted the plane, potentially causing catastrophe. Mr Bailey is a former RAAF pilot and is now a commercial pilot who has flown more than 26,000 hours during a career spanning 45 years.

However, Rex deputy chairman John Sharp said Mr Bailey’s assertion was “emotive and completely unfounded”.

“Further, the propeller only separated when the first officer selected ‘fuel off’, which also feathers the propeller,” Mr Sharp said.

He added that Rex had exercised an “abundance of caution” and replaced the gear boxes and shafts of five aircraft which were of the same series as the separated propeller.

Mr Sharp also disputed the claim by Mr Bailey that the gearbox was a weak link in this type of engine and had been linked to the cause of the incident.

“The General Electric engine fitted to the Saab 340 is a free power turbine and as such, the engine and gearbox have no mechanical connection as most industry professionals would be aware,” Mr Sharp said.

The ATSB report did not find that engine overheating, caused by problems within the gearbox, were linked to the loss of the propeller, as Mr Bailey had suggested...

...Botswana O'Hooligan provides IMO a comment worthy of a QOTM nomination and a choccy frog to boot...

Botswana O'Hooligan

2 days ago

There is a law called Murphy's Law and it states that anything that can go wrong, will go wrong, particularly where aeroplanes are concerned. Elsewhere in this newspaper today it has been written that a 747 had a stick shaker activate when on descent. Stick shakers activate when the machine is about to stall aerodynamically, that is when the angle of attack of the airfoil approaches about 15 degrees to the airflow. Apparently, or so they say, this took place when they were no where near a critical angle of attack but the orange boxes containing the flight data recorder and the cockpit voice recorders will no doubt shed light on that.

GE probably thought that the prop mounting flange was there forever and there was no need for an inspection.

One night a fourteen feet in diameter prop on the machine in my care went into what the Poms call -auto coarse pitching- tho we call it -auto feather- among other things unprintable, for no reason at all, and the manufacturing people in the UK said in a hurt tone, "old chap this cannot happen," but about a month later in a different aeroplane the same thing happened again. The Poms were horribly upset for it just cannot happen they wailed.

Another night ex a joint called Midway island for Adak in the Aleuts, when passing through thirty thousand feet, bitching Betty told us that we were far too low and we should pull up immediately and initiated a pull up manoeuvre on long suffering "George," aka one of the auto-pilots. Since Everest is lower than that, Betty was having some kind of hissy fit.

All the above examples are simply demonstrating Murphy's Law and if anything can go wrong, it will, and that's why, even after being a professional pilot for fifty five years I am only scared of three things even in retirement, heights, women, and aeroplanes.

 


MTF...P2