Guest Maj Millard Posted August 31, 2015 Posted August 31, 2015 Found this particular unit badly cracked today during a 100 Hourly inspection. TT appears to be 1000 hrs plus, prop was re-torqued 100 hours previous, and has cracked since. I will be forwarding a full defect report to the Tech manager ASAP ....please remove your prop-spinners if fitted, and look closely at all alloy prop hubs during inspections......only the foward facing unit found cracked. Although there is surface corrosion present ( it's a coastal aircraft) I don't feel it has contributed to the cracking in this case.
Guest Maj Millard Posted August 31, 2015 Posted August 31, 2015 I think there could be a few of these out there Frank..........and by the way the owner and I definitely decided this was U/S , and it has of course been removed from service.
Guest Maj Millard Posted August 31, 2015 Posted August 31, 2015 any vibration coming from the prop? A run up and mag check was conducted prior to the 100 hourly and compression check, operation was normal with no roughness observed.
Head in the clouds Posted August 31, 2015 Posted August 31, 2015 Crack propagation is an interesting subject and via the study and experiences of cracks in rotor blades, my LAME taught me quite a bit about them. The nice thing about them is that they propagate in a fairly linear fashion, so if you 'map' them by noting/marking the extent of a crack, and the date/hours in service, and then compare that a bit later, you can predict quite accurately where it will have extended to in a given number of operating hours ahead. The cracks you show Maj, are quite a bit different from the norm, in that they have formed from the bolt side, whereas I would have expected any load cracking to have come from the forces applied by the blade itself, and so to have formed near the axis of the blade. My guess, therefore, is that the crack would have developed very soon after the re-torquing last 100hrly, since that's probably when the increased load/force was applied. If that's correct then it's a fine testament to the design of the hub that it held onto the blade through the last 100hrs and so many cycles. Thanks for posting Maj, very valuable info for sure! And in future I'll be having a look under the spinner perhaps an operating hour or two after any re-torquing of the hub/blades.
facthunter Posted September 1, 2015 Posted September 1, 2015 Fairly extensive cracking. Probably didn't happen yesterday and propagated (no pun intended) over a fair period of time. The sort of thing that should have been noticed on a thorough preflight. I suppose you have to know where to look... But.... not hard to see. 1,000+ hours is a good run.. Nev 1
Head in the clouds Posted September 1, 2015 Posted September 1, 2015 Fairly extensive cracking. Probably didn't happen yesterday and propagated (no pun intended) over a fair period of time. The sort of thing that should have been noticed on a thorough preflight. I suppose you have to know where to look... But.... not hard to see. 1,000+ hours is a good run.. Nev True Nev, but how many people remove the spinner to look at the hub as part of a pre-flight? In fact most folk aren't allowed to do so unless they're an L2 or the owner/builder, are they?
facthunter Posted September 1, 2015 Posted September 1, 2015 Does it run one? They flew the DHC-1's with no spinner so as to look for cracks on the Fairey Reid metal prop. Nev
Head in the clouds Posted September 1, 2015 Posted September 1, 2015 Does it run one? They flew the DHC-1's with no spinner so as to look for cracks on the Fairey Reid metal prop. Nev Well, I don't know if that particular aircraft has a spinner or not, perhaps not, judging by the rusty bolt heads, but if not then you'd have thought the cracks would have been noticed earlier. My point was that the majority of 95.55 aircraft these days do seem to have spinners, as mine will, and the spinner would cover the hub and prevent the cracks being seen however meticulous the pre-flight inspections. And - unless owner/builder or L2 the pilot shouldn't remove the spinner to check. Hence I'm glad this has come to my notice as I have the new extra large hub to swing a 74" 3 blade Bolly so the pitch will be set quite fine and consequently plenty of thrust load on the hub from the long blades. Also I'll be very careful not to overtighten the blade clamping bolts which is quite tempting to do since the proper torque (about 110 inch pounds from the Bolly instructions IIRC) never seems quite enough ...
facthunter Posted September 1, 2015 Posted September 1, 2015 Yes, they would be very dependent on the correct torquing. I reckon they are fatigue related and a wonder they didn't let go. Spinners look good but they crack up faster than props in many cases.
Guest Maj Millard Posted September 1, 2015 Posted September 1, 2015 If you look at photos 1 and 3 it is the same end, so that crack actually travelled in three directions. I suspect the inner crack parallel with the hub probabily started first weakening things ( it looks the oldest) and then the outer cracks developed. Don't look like it would have ended well if left undetected much longer. The foward cracks radiating from the blade hub hole should be plainly visible during a preflight even with a spinner fitted, but you have to take the time to look. This aircraft has had a spinner fitted most of the time. I've worked in GA for over forty years so have seen all types of prop related cracks over the years, especially on alum spinners and their alum backing plates. I know my Lightwing would look great with a spinner but I've never run one as I like to look at my prop hub every now and then.
eightyknots Posted September 1, 2015 Posted September 1, 2015 I am glad you spotted this Ross. You have probably prevented a near-future calamity! 1
eightyknots Posted September 1, 2015 Posted September 1, 2015 Does it run one? They flew the DHC-1's with no spinner so as to look for cracks on the Fairey Reid metal prop. Nev I've worked in GA for over forty years so have seen all types of prop related cracks over the years, especially on alum spinners and their alum backing plates. I know my Lightwing would look great with a spinner but I've never run one as I like to look at my prop hub every now and then. I have always wondered about the value of a spinner. Consider: 1. How much better is it aerodynamically when there is a cowl just behind the prop at that point which is not very aerodynamic either? 2. Is the extra weight of the spinner compensated for by the aerodynamic improvement? 3. We have already seen the value of having no spinner to check for propagating hub cracks. Is the principal purpose of a spinner purely aesthetic?
Head in the clouds Posted September 1, 2015 Posted September 1, 2015 Is the principal purpose of a spinner purely aesthetic? No, depending on the application it makes an immense difference to the airflow into the cooling ducts, and this is particularly important during the high-power, low-speed climb after takeoff. Some types (low climb speed STOL types for example) will overheat very quickly in the climb if the spinner is not fitted. I don't think spinners make any difference in the cruise, they're not a 'streamlining' thing unless the airframe nose is quasi-circular and the spinner diameter matches that, like the Bugatti perhaps... in which case the spinner has little to do with the cooling aspects and much more to do with reducing form and friction drag. 1
Keenaviator Posted September 1, 2015 Posted September 1, 2015 Looks like intergranular corrosion to me. The prop hub halves have been machined from a block of aluminium and machining has cut the grain causing stress risers.
jetjr Posted September 1, 2015 Posted September 1, 2015 How many hours in service? I have Bolly hub but its anodized and sloghtly different
Guest Maj Millard Posted September 1, 2015 Posted September 1, 2015 We think 1000+ hours TTIS...it has been on a couple of different aircraft so bit hard to track down conclusively. I don't think intergranular has initiated the cracking at all...I am very familiar with intergranular...see it all the time. It appears on some of the older cracks that intergranular may have been kicked off by moisture entering the crack after initiation. The later Bolly 2-blade hubs are of a completely different design and gold anodised.
Head in the clouds Posted September 1, 2015 Posted September 1, 2015 Are there any records detailing the torque to which the bolts were set 100 hrs ago? As I mentioned previously, and if my learning is correct then cracks always propagate progressively, so a sudden change starting from and only during the previous 10% of the hub's life indicates something has changed dramatically. Whether intergranular corrosion played a part, I'd have to say probably, because intergranular stress is a certainty, given the likely/almost certain billet orientation and the loads applied by the bolts against the butt of the blade. If I had to provide an opinion from what I learned dealing with rotor blade cracks, I'd have to suggest that the bolts might have been over-torqued last service. Note that there is a gap between the hub halves so there is nothing to prevent distortion of the hub reacting between the bolts' pressure against the 'solid' blade boss.
jetjr Posted September 1, 2015 Posted September 1, 2015 Latest versions are thicker and have just a flat lamd for bolts only Torque on blade bolts is 11 and hub just 13 Ive found Bolly guys offer excellent support
Head in the clouds Posted September 1, 2015 Posted September 1, 2015 Latest versions are thicker and have just a flat lamd for bolts onlyTorque on blade bolts is 11 and hub just 13 Ive found Bolly guys offer excellent support I couldn't agree more, I'd say Peter and Craig have to be the most supportive aviation component suppliers I've dealt with, in 40yrs of sport and commercial operations. I'm sure they'll provide valuable insights into what actually happened - far better than our speculation. 1
Guest Maj Millard Posted September 1, 2015 Posted September 1, 2015 What does Bolly have to say about it? Don't know yet....
Guest Maj Millard Posted September 1, 2015 Posted September 1, 2015 I always torque per manufacturers instructions....don't about other people in the past. Pilot owner could have also re torqued during past 100hrs. Internal crack is much older.
jetjr Posted September 1, 2015 Posted September 1, 2015 Just my point of view but I reckon manufacturers deserve to make good a problem before their product or brand is damaged on forums etc
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now