RNZAF Strikemaster wing cracks

[Dave Homewood]

I'm curious. How were the cracks actually discovered in the wings of some of the RNZAF Strikemasters?

Was one in for major servicing and they were discovered and then the rest checked? Or are there regular checks done on the line for such things? Or was it the result of an investigation overseas that all had to be checked?

Which aircraft were affected? And what was the repair process? I recall being told they didn't push them as hard in the latter days as they had earlier on, to preserve the wings, did that apply to repaired/replaced aircraft only or the whole fleet?

Also when were the cracks actually discovered?

[Dave Homewood]

I also meant to ask, have other RNZAF types suffered from fatigue and cracks resulting in such drastic repairs/mods? Did the Orions ever have cracks discovered in their wings before they had replacement wings put on?

[beagle]

Mainly corrosion in the P3 Dave. Remember a photo of 3 guys during a group servicing with their heads sticking out of the front of the wing spar inboard of no#2 eng while they replaced a section of it.
Pretty sure minor cracks were found during group servicings and repaired as per correct techniques.

[guest]

There was another thread on here about the blunty cracks; they were discovered by chance and it is thought that if the aircraft had flown again, it may well have been its last flight.

Non-destructive Testing (NDT) is used to to find the likes, I will let a framey lead on the methods - die, x-ray, etc

P3 was spar cap delamination and highest fatigue life world-wide that led to wing replacement. There are holes in the leading edge spar for all sorts of things including fuel pumps, etc for the wet wing.

[classicman]

Hi guys here is a photo I posted a while ago. It shows the crack inspection holes drilled into the old girl's wings. To keep the wings on we were limited to +3.5\-1.0G for her final years. There were large placards in the cockpit reminding you of this and the primary instrument during aeros was the G-meter!

[curtiss]

Wing cracking in the Strikemaster was in three major areas: Upper wing attach lug, lower fuselage spar boom and wing cleats. The silver tape patches on the wing were to allow repetetive inspection of the cleats as it was allowable to fly with a certain number cracked .

The upper lug crack issues resulted in new wings being fitted to some aircraft and a program was started to replace the lower fuselage spar booms, but only one aircraft was done before it was decided to replace the aircraft.

Some of the cracks were discovered by RNZAF staff and some were discovered by PACL who were also carrying out Strikemaster servicings.

Strikemasters had servicing based on flying hours and fatigue index units ( FI). The consumption of FI was calculated from the fatigue meter and actual FI was based on an algorithim that was developed following extensive instrumented test flying managed by the Defence Scientific establishment. From memory, there was a major FI servicing at 76 FI.

Inspection requirements were continually updated from BAE , RNZAF and other operator data. It would be fair to say however that RNZAF fatigue usage was the highest in the world due to both the nature of turbulance over mountainous areas of NZ and the RNZAF use of the Strikemaster for FLIT ( Fighter lead in training).

[trimotor]

Having been on Wings course when the cracks were first noticed, and then flown the aircraft as a QFI, the cracking was a problem: each aircraft had its own folder with drawings if the known cracks, which all had to be checked periodically, as well as if the revised G limits were exceeded.

Different G limits were applied, depending on the severity of the problem-some aircraft were '3 G Jets', useful for instrument flying and high navexes, others were '4.5 G Jets' which were carefully used for anything and those that had new wings ( which were rare, as they cost NZ$1m in the mid-80's). By the time I was QFI-ing on the Blunt, there were no restricted jets (normal +Ve G limit was +5.5).

[Dave Homewood]

Cheers for that Trimotor.

[tbf25o4]

the Iroquois fleet suffered from severe tailboom cracking especially in the late 1980s and early 1990s that resulted in
having to upgrade the tailbooms. before they went to Belisi, additional strakes were added to the tailbooms to reduce
the flexing and therefore the chance of cracking

[Dave Homewood]

Very interesting Paul. What was Belisi? Is that in East Timor?

[trimotor]

While not having flown the Huey (well, not qualified to fly it...!), though I know a few guys very well wh did, I think the strakes were not so much for strength as for airflow control, which dramatically increased the effectiveness of the tail rotor.

[noooby]

Correct trimotor. Induces Coanada effect, turning the tailboom into an airfoil, offloading the tailrotor, especially when in the hover.

[Pete Croft]

Jan 8, 2012 19:17:43 GMT 12 Dave Homewood said:

I'm curious. How were the cracks actually discovered in the wings of some of the RNZAF Strikemasters?

Was one in for major servicing and they were discovered and then the rest checked? Or are there regular checks done on the line for such things? Or was it the result of an investigation overseas that all had to be checked?

Which aircraft were affected? And what was the repair process? I recall being told they didn't push them as hard in the latter days as they had earlier on, to preserve the wings, did that apply to repaired/replaced aircraft only or the whole fleet?

Also when were the cracks actually discovered?

Old thread I know, but FWIW I was on squadron at the time as an Avmech, so it must have been around 1985. I'm pretty sure the cracks were picked up on a flight line check, which would likely have been a routine Fats and Oils, or a post flight sanity check after some knuck pulled excess G or put it down a little hard.
I rememember that BAE had an engineer over pretty smartly, and at least one of the aircraft got
wired with strain guages and a data logger - all the data had to be sent to the UK by dialup modem, which was a pretty trixy piece of kit at the time ;-)
The cause was put down to the almost continuous low level G due to turbulence IIRC. Supposedly the skies over Europe are less bumpy than ours, so this had never been factored into the fatigue life calcs.


Pete

[Dave Homewood]

Thanks Pete. Welcome to the forum.

[suthg]

That's true as an engineer, we have to design for a fatigue life of 100,000 hrs so that if the material strength is high enough, that the maximum stress of say 40% yield is an infinite life, then either the maximum load to incur the 40% yield is avoided by operation control, or you use a larger section of material or a higher yield stress material to get the infinite life.

That's all well and good in the civil stationery application where weight (or dollars for higher spec material) doesn't matter such as in a Pulp Mill or a big Boiler (ours are 70m tall) but in a production military aircraft, weight is a factor, and so is cost to some degree, then the designers compromise on material, on weight with a lot of removed material in the central vertical axis of a spar where it has little effect on strength, to the detriment of torsional strength, but you may end up with a fatigue life of moderate stresses (loads) causing cracks without having any serious excess G's. Or if there are some overstressed situations, then they accentuate the ability to cause a crack and even continuous moderate stresses can activate a crack and propagate it into the flanges where the real strength should lie because the original cross sectional area has already been compromised by a minor crack through a single overstress.

Perhaps I have exaggerated the situation or even generalised it some, but to a materials design engineer, then the fatigue life is something you try and extend by good materials, good design and bracing and fixings. Otherwise you are up for regular checks for cracks and even a specific stated end of life hour limit.

[delticman]

The Strikemaster fleet were cycled through NZ Aerospace Industries. I'm not sure if I have any dates in my notebooks as they don't have hoppers in them. The wings were removed and I think new castings or similar installed and then the holes were rebored with a larger hole and larger bolts installed.

[kalz]

I actually found the first crack in the Blunty whilst looking for a fuel leak (post overstress) on the line at 14.

I removed the lower fillet fairing as it appeared the leak came from the inbd tank feed and noticed a shinny bit on the lower edge of the spar.

This in itself was not unusual as we had chaffing from fairings but the drop of fuel had a dark line thru it. Thinking it was dirt I cleaned it and it did not go away. Grabbed a mag glass and there it was a crack emminating from a small fretting area. The rest is history.

[Dave Homewood]

Well done Kalz! And welcome to the forum.

[baz62]

Well done finding it, who knows how much longer it might have been til something catastrophic happened. I recently found an online source for the RCAF Safety magazine starting from the 1960s and its interesting how many things have been found by tradesmen looking to fix or replace and notice another. In some case going much further than the original task entailed and finding a bigger problem (which in a lot of cases was duplicated in other airframes of the fleet type. I think sometimes it shows the character of the man (or woman) who doesn't just do a job but thinks"well while I'm here I'll have a good look around" and thank goodness they have that work ethic.

[baronbeeza]

Reminds me of a cracking issue discovered on the F-27. During an engine change the firewall mounts get checked with a magnifying glass. One of the mechs came in and reported to the SNCO's that he thought he had seen something.

Bikie sent Murray J (Tech) out to have a look. A few minutes later he came back in and said 'Yep looks like it, Top inboard mount'.

The mech was still in the the office and he looked up...... 'Nahhh, Top outboard' !!

One engine, two cracked mounts.

That started a chain that went through the RNZAF fleet, onto the Air NZ and eventually the world wide fleet. Our cracking wasn't the worst and we had to contend with a priority system to access the (very) limited spares.

Welcome Kel, I suspect we have shared a beer in the past.