Club Homepage Forums Classified Ads Buy & Sell - Everything But Airplanes Metal Fatigue??

In the past 6 months or more I have been looking for a nice 150. However, I have a question about the age or condition of these planes and what we should be looking for in terms of "Metal Fatigue". I am aware that in the past there has been discussion of "how many hours is too many hours?" That discussion was topped off with mention of the plane in Australia with umpteen engine changes, and 20,000 hours + as an example of how long some have gone. However, almost all who have looked for a 150/152 know that some of these planes are getting a "little long in the tooth." 5000 and 6000 hours is quite common. Under 3000 hours is getting to be unusual. Which raises the question, what should we as a group that like and fly these planes be looking for in terms of problems other than obvious cracks in the frame. Many are aware of the tail cone "cracks" that has arisen, although few have actually reported it as being a real problem. Those of you who are A/P or whoever, is it possible to come up with suggestions for a "check list" of what and where to be on the look out for? If possible, what should be included on it?

Bill

I'm not an A&P, but I have SOME experience fixing 150 problems. One common problem is worn flap tracks. Lower the flaps and feel for slop. The next is the main landing gear box. A good indicator of its heatlth is to look at the row of rivets just below the landing gear leg. There's about 5 on each side. Are they making smoke? If so, it's probably a good idea to get it repaired, and may be an indication of other landing gear issues.

Let me start by saying that metal fatique is not a common issue with Cessna aircraft in general! The most common metal fatique problem I'm aware of on 150's and 152's concerns the floor pans where the seat rails are attached. These fairly often are found with cracks radiating away from the seat rail mounting holes, usually the front ones, and are easily detected with the carpet removed for inspection. Caught early, a simple full length patch under the seat rails will normally restore full strength and integrity here. Yes, there are the service letters and AD's for the nutplates in the tail, which sometimes identifies cracked mounting brakets during inspection. Most of these should have been corrected by now. The earliest 150's sometimes have expensive to repair landing gear bulkhead problems, but these are easily detected through floorboard inspecton panels, and many have had reinforcement kits installed (see Charles Hanna's website [150cessna.tripod.com] for details). I've not heard of any other metal fatique problems common to any 150's or 152's regardless of age or flight hours. They are safe airplanes that are not going to come apart in normal flight!

Of more concern to me is the corrosion that can be found on some Cessna singles in the front spar caps. Made of extruded aluminum, and absorbing stresses from wing loading, the spar caps are more susceptable to serious corrosion issues than other areas, and can be costly and time consuming to repair. The carry through spars that connect the wing spars together through the cockpit are also an area of concern, as is the area of the front and rear spars which mount the wooden fuel tank spacer blocks, which can trap moisture and promote corrosion.

These issues are not so common that every airplane needs disassembly and inspection prior to purchase, but I would satisfy myself that the previous owner and mechanic are aware of these issues and that these areas have been inspected in the recent past. If the purchase comes with a fresh annual, you want these areas inspected. A pre-buy inspection would not normally go deep enough to identify these rare enough but expensive to repair problems.

Club member Mike Arman's book "Owning, Buying or Flying the Cessna 150/152" [cessna150-152.com] is a good starting place for the first time Cessna 150 or 152 buyer, and is available from the Club's store. Continue searching the forum and gathering information, and don't hesitate to ask the forum any question that comes to mind. There is little that hasn't been researched before, and there is always somebody here with a friendly answer. We are all licensed to learn, here!

Carl, and All,
First thanks for the answers and anyone else who has input, feel free to jump in. The issue of corrosion probably should go together with metal fatigue. Carl you are, in my opinion correct about the corrosion and to my knowledge there has not been any in-fight break-up of a Cessna under normal circumstances. (discounting getting caught in a thunderstorm--then all bets are off.) The design is strong and apparently well built. My idea in this thread was to get what the membership have come up with and I would still like for all to contribute so that newer members can benfit. I know some of these topics have been beat to death, and like a wheelmark keep coming up. But, I would rather discuss too much, review, and be safe than to keep quite and hear about someone digging a hole with a plane. Thanks again.
Bill

Talking about metal fatigue brought to mind an airplane I have been working on the past couple of days. It is a Boeing 777-200 and was delivered in (I think) year 2000. We are in the process of conducting its most exhaustive check to date, and I noticed in thw work center, the hours and cycles that are on it. I do not recall the exact numbers, but it was 32,000+ flight hours, and 5200+ cycles! Now we flew DC-9's for 30+ years and I think the high time aircraft accumulated something around 70,000 hrs, but the cycles were something close to that also. Cycles on a large airplane are the defining factor, due to the stresses placed on all parts of the pressure hull by the pressurization system. Still, this aircraft is as clean as a new one, and I've been amazed at the amount of composites used in it. The horizontal and vertical tail structure is almost all composite.

Charles

Chuck,

Read your note having to do with cycles of the big planes having to do with pressure expansion, contraction etc. Can you translate that to a 150? Obviously, pressure expansion is not a factor. I would think cycles of take offs and landings and flexing of the landing gear would be the major area along with flexing of the wing spars in flight. is this correct or are there other places where flexing would be a factor, ie. rudder, stabilizer, motor mount?

Bill

The doubler / bulkhead in which the leading edges of the tail feathers bolt into, comes to mind. Considering I had to replace this unit in 72G. But the stresses here usually are created from ground handling the aircraft by pushing and pulling on the tail feathers. Not by stresses of flight.


The Cessna landing gear is almost bullet proof. Many of my landings have gone to proving that point!

And I would think engine mount stresses would be taken up in the rubber mounts themselves on the most part.

I just cannot think of too many places that give and flex a whole bunch in the Cessna 150. One's first experience with turbulence usually is enough to convince most people of this fact! (Note: Do not wear base ball caps with the button on top while flying the 150/152 in rough air!)

Is that the voice of experience I hear?

I remove the buttons from baseball caps I wear while flying to accomodate the headset. However, I too have hit my head on the head liner in turbulence.

Chuck, by cycles, do you mean where the plane is presurized and depresurized, or is it something else?

Ooops, I hear my phone ringing, been kinda out of it. Let's see if some distracting meandering can help us out here, Bill_. I can add some ruffles and flourishes to some points, at least.

Corrosion (more than fine surface stuff) is the greater problem, as has been stated, because pitting makes for crack starters. Beyond that, the service and maintenance history are the real considerations.


Not really comparable; usually there is one pressurization cycle per flight and it only stresses the pressure vessel. Flight, etc. loads stress the whole airplane, in various and changing ways over many times in one vehicle operation.


All this, and more! The honest truth is: the whole airplane, it all flexes to one degree or another. Different maneuvers/inputs produce their own "hot spots." On the F-2 that I work with, the same part may have multiple critical load cases (some in-flight, some by other events), depending on location and the failure mode. And we have an incredible number (that I can't remember right now) of load cases, being a military combat aircraft. But wait, there's more! Vibro-acoustics can produce fatigue as well, on the 150/2 from the motor vibration and the propeller pulses. (The F-35 STOVL gets beat up pretty good by the ground reflections of the vector thrust)

One of the advantages of a large fleet with substantial time in service is the structure is pretty well proven, the major and most minor hot spots identified (an unsung role of the mechanic is this kind of feedback of what is seen in the field). Thus what you are looking for is really just making sure those items, mostly AD's, are up to date. The structural design has several safety factors to handle the limit load stress levels with reasonable room to spare. This means there is a lot of potential life in an aluminum structure. And with this margin, most of the time in conventional structures a crack weakened part will shed load into other structure and the crack grows slowly. This gives you time to find the crack in normal maintenance. And as the other structure overloads as the crack unloads, it yields and bends and gives you further warning of the crack. (Assumes loads are within limits, although overloading can do the same bend-before-break thing if not excessive - the 152 will show skin wrinkles near the MLG if landed in a golf course sand trap, for instance . . . ) This wider range of elastic-plastic behavior of aluminum is part of what makes it so useful in aviation. Not 100% foolproof (it's a human endeavor), but has worked rather well. Pressure vessels are loaded more uniformly and respond differently, hence the separate tracking of pressurization cycles from flight hours.

I guess this is the long way 'round to saying that fatigue, while important, is not at the top of the worry list here, in the GA fleet. There is some growing concern for the very high time/cycle airframes (principally the military and commercial air transports) just because it is new territory. As development times have lengthened and costs have risen, aircraft fleets have not been upgraded as quickly, which used to put most of the "old-timers" out to pasture before this was a significant issue. In addition, aluminum has a peculiarity in that a low stress over a long period of time can cause material failure, too, so there are definite (but high) ultimate time limits to some (parts of) aluminum structures.

From my perspective, I consider "low" airframe time more like a cosmetic issue, nice to have but does not really affect the function now or future. At annual/pre-purchase inspection, 80Q turned out to be in rather nice shape for a 25 yr, 9400+ hr airframe, not to say it does not show it's age and use. Spent 20 of those years in the arid climes doing club and probably some rental trainer duty and having regular maintenance done by the same person for years at a stretch, before coming down here to slumber for awhile until I came along and woke it up. Two recorded events, neither of which produced any airframe damage we could find in inspection (but did engender two engine swaps, one change of owners, and the switch to the Sensenich prop ). Some might be put off by the total time, but I am still shaking my head at my luck in the whole deal.

(OK, more meandering than usual. )