Fwd Carry Through Spar FAILURE

[Poncho73]

The initial inspection was carried out to investigate Inspection Report
Cessna 170-B
Serial # 20908
C-FEER

C-170B, C-FEER, Ser # 20908, TT 2,600 hrs

The initial inspection was carried out to investigate buckled and distorted fuselage skins at the lower strut attach point after an inner tube failure on landing at CYFD July 21, 2012
Further detailed inspection revealed that the forward carry through spar # 0511162-5 had failed inside the LH upper door post assembly # 0513006-44.
It is not possible to inspect the area where the spar failed without removing the wing and spar attach block fitting # 0411129.
The aircraft inspection program on this aircraft has included an intensive corrosion inspection and prevention plan for the past 35 years. At this point corrosion does not appear to be the cause of the failure. It is suspected that the initial crack had been present for some time. The inner surfaces of the carry through spar remain heavily coated with LPS-3 from when they were treated during the last wing removal (1975)
All recommendations of the International Cessna 170 Association for inspection of this area have been carried out during numerous inspections with no discrepancies.
Suggestions include cutting a 3/8 in hole through the fuselage front spar cap plate # 0511000-24 just inboard of the carry through spar attach blocks. This would give a good boroscope view of the inner spar surfaces where it passes through the door post. This aircraft has inspection holes cut in the cabin roof skin over the rear carry through spar ( boroscope inspection is part of the 100 hour inspection for this area) It had always been planned to cut inspection holes for the forward spar but concern over sealing the holes from moisture ingress held this up. An X-ray inspection would discover cracks in this area but suggest costs would be high.
I consider this a very significant failure considering the amount of common 100 series Cessna aircraft around the world.
As the area is opened up further information will be provided.

An SDR (Service Difficulty Report) has been filed with Transport Canada

WhenI figure it out I will include photo and diagram


Les Edge

AME # M095846

Thanks Les glad you posted. I will swing by Burlington this weekend, I was north all weekend so RYX was tied down at Brampton. If you need me to put anything thru the lab (SEM) I can get it thru for you. Cheers Dave

[Poncho73]

I flew over to Burlington in RYX to pick up the carry-thru spar from Les. Next week I'll have a look at the fracture surface in the scanning electron microscope (SEM). I will try to post some quick photos....need to resize first. Cheers Dave

[voorheesh]

I investigate aviation accidents and have been on two cases of inflight wing failure (CE210 and V tail Bonanza). While those accidents were totally different in nature from this one, what I learned is that wing spar failures occur near the wing attach point and excessive in flight load factors are common in these type cases. The CE210 and Bonanza both started with in flight empennage failure followed by extreme nose down pitch. Both wings fail in a downward direction. Spars snap near attach bolts. The failure shown in these photographs indicate the possibility of an inflight event such as sudden severe turbulence. The tire failure may have caused some vibration and side loading but it most likely would not have been significant enough to cause displacement of this spar component. An unusually hard landing may also be a possibility but one hard enough to cause wing damage like this will almost certainly result in landing gear and lower fuselage damage/failure (I have investigated more than one single engine Cessna in accidents involving hard landings). I recall a Cessna 402C back in the 1970s that flew through B747 wake in a perpendicular direction resulting in a huge bump. A passenger not tightly belted broke his neck. XRay inspection discovered wing spar damage. Both engines suffered crank shaft failure. The pilot did not recall it being that bad an event. The airplane flew nearly 10 miles to landing after the event with no problems and made a ferry flight to an airport where the inspection took place.

Engineers out there correct me if I am wrong, a CE170 carry through is subject to tension forces when the airplane is supported by wheels, vertical shear load from the fuselage gussets, compression, shear, and tension while in flight. I believe that a significant inflight displacement occurring within 1 or 2 seconds causing up to 6 or 7 gs might result in a crack/failure as shown in pictures (all 3 stress forces acting on the metal almost instantaneously). If the metal has any internal corrosion (in some cases not detectable by the eye even with a magnifying glass), will make the failure even more likely, given the large force. The pilot may not even be aware of what he/she just encountered and the airplane could continue to fly months/years in that condition until the next high g event where the wing will probably fail. In this case he/she was lucky or had an angel riding along as copilot (maybe both).

Airplanes experiencing high load factor events need to be inspected. For big airplanes, the factory maintenance manuals specify how to do it. For small airplanes, experienced maintenance organizations can provide almost the same level of expertise. One comment in this story regarding the drilling of a small inspection hole into the spar structure made me wonder a bit. The writer used the word "suggested". Who suggested this? Cessna Aircraft? An STC holder? You can't just drill a hole in a spar unless you have engineering data. Maybe I did not read that part right.

Finally, I suggest you ask Canadian accident investigation authorities if they can investigate this. They can certainly give you an answer on the corrosion question and it has been my experience with NTSB, their engineers can calculate likely load factors by analyzing failure signatures. For example, an NTSB engineer can look at those pictures and tell you which direction the force was applied or whether it failed on its own. Given the number of these Cessna airplanes, as you point out, they might be interested in finding more about this unusual case.

[wingnut]

This type damage can cause the carry thru channel to crack..................years later if its not replaced. I'm not an engineer, but the way a 100 series wing is built, (especially early series), would fail about 18 inches inboard OR outboard of the lift strut attach point before the carry thru would fail (assuming in flight g's)

The OP subject airplane has been previously damaged by something other than in flight turbulence, and was missed during repair and all subsequent inspections. I wish I was able to say I find this hard to believe.

Well, I was going to post a picture. It was to big. The picture was of a 172 that ran between 2 trees and made it a sweptback design. If damage causes the flap to contact the fuselage fillet (most ground loops, even some bird strikes to the outboard leading edge) then the forward carry thru has been tweaked. It may not appear that way to the eye, but it has been "sprung". That carry thru channel is cheap.

[GAHorn]

EXACTLY, Del,... This has every indication of earlier damage that was either unnoticed, or deliberately
ignored. This did not simply "happen" due to the light corrosion seen in these pics.
I would not encourage anyone to be cutting holes based upon this example.

[Bill Hart]

This type damage can cause the carry thru channel to crack..................years later if its not replaced. I'm not an engineer, but the way a 100 series wing is built, (especially early series), would fail about 18 inches inboard OR outboard of the lift strut attach point before the carry thru would fail (assuming in flight g's)

The OP subject airplane has been previously damaged by something other than in flight turbulence, and was missed during repair and all subsequent inspections. I wish I was able to say I find this hard to believe.

Well, I was going to post a picture. It was to big. The picture was of a 172 that ran between 2 trees and made it a sweptback design. If damage causes the flap to contact the fuselage fillet (most ground loops, even some bird strikes to the outboard leading edge) then the forward carry thru has been tweaked. It may not appear that way to the eye, but it has been "sprung". That carry thru channel is cheap.

Thanks for your post Del,

What I am currieous about is, is there a good way to inspect the carry thru with out removing the wings. I just replaced my windshield and head liner and inspected the carry thru as best I could but would love to get a look at the inside. With your experience can you describe your method of inspection in this area?

[Poncho73]

Just comment to all on this investigation. I'm a certified aircraft accident investigator and have the part in hand. I will be looking closely at the fracture surface in the SEM. At this point, I will not comment on the failure but I will say I see no immediate panic in changing any current inspection requirement. Transport Canada has been notified by the aircraft owner. I will provide an update to the progress as it moves forward.

[hilltop170]

Cessna airplanes with optional lifting eyes already have a 1" to 1-1/4" dia hole in the cabin top skin directly over the spar-to-wing attach block. Removing the lifting eye may provide enough clearance to insert a borescope. Some of you maint types may know if there would be enough room to see anything. If nothing else, the hole would allow access for spraying Corrosion-X or ACF-50 into that area for corrosion inhibition.

C170 Lifting Eye_1.JPG

[wingnut]

I've only used boroscope for corrosion. I doubt that it would be useful for crack detection. (sounds kinda nasty don't it). The crack in the OP's picture would have been visible with the headliner pulled back. In fact any crack that could develope would be easier seen externally. The only areas not visible externally would be the vertical flanges behind the door post bulkhead. I can't imagine a crack propogating in that area, and a crack propogating into that area would be visible on the horizontal flanges and bottom surface of the carry thru channel first.
The crack in the OP's picture is inboard of the forward doorpost bulkhead mating area, and therefore would/should have been seen a long time ago. I would like to see a picture of the cabin top skin in same area.

[Poncho73]

Guy's I have finished up the lab work on the carry through. More on that later. FYI and for piece of mind, I see no reason to add any special requirements to what is already available to all of us. Go flying, no worries.

[Les Edge]

Re some of the above comments

Aircraft owned by myself 39 years, 99 % flown by myself

Very little grass strip use

I have been a licensed Aircraft Mtce Engineer for 45 years ( same as an A&P)

Aircraft has not been involved in any exceptional hard or heavy landings in 39 years ( TT 2600 hours)

Aircraft has never been groundlooped in 39 years, I have seen the results of numerous of these, lots of damage on some, but no spar failure

No incidents or exceptionally heavy turbulence encounterd in this time

Exterior visible portion of the carry through spar inspected approx 20 hours and 1 year ago

This aircraft is inspected and maintained to a farily high standard

The interior surfaces of the door post are in good shape, minimal corrossion

THIS FRACTURE IS / WAS NOT VISIBLE. The only possible way to inspect this area is with the wing and blocks removed,

Re the lifting ring, you cannot see inside with this removed, block must be removed

Cabin roof and fwd spar cap undamaged and in good shape. Six rivets sheared on the aft side of the spar cap over the door post during the incident, where the door post moved outboard with the small section of the fractured spar

Re the comment on the rear window - the flaps contacted the rear window during the incident, first time

I don't know how long the spar had been fractured but it definetly should not have failed during a tire failure event

No matter what -- Every 100 series Cessna that I am involved with will be inspected in the door post area with the addition of two holes in the forward spar cap. I will advise how the paperwork for this moves along.

I have major concerns with this wether it is a one off or not

Dave Fisher will be posting the results of the fracture analysis

Les

[jatkins]

I heard a rumor that Transport Canada, was having a follow up meeting on this subject ?

Any updates out there ?

[Poncho73]

I heard a rumor that Transport Canada, was having a follow up meeting on this subject ?

Any updates out there ?

Interesting you mention this as I have been trying to set up a meeting with TC to close off the file. I plan to hand over everything to TC then, results to be released after their blessing. Dave