WHEN TO REPLACE A LONGERON?
By John Seevers
I have been building P-51 longerons since 1982 and believe my longerons are in most of the flying mustangs today. Based on this experience as well as my professional background as a PhD in Engineering with 29 years as a university professor, I offer the following as my observations, opinions and advice dealing with the replacement issue for longerons.
The P-51 Structural Repair Manual is the governing authority for damage repair and airworthiness determinations for this aircraft. It is apparent that these planes are still airworthy with significant structural damage even for the demanding use as a combat aircraft. This provides great comfort and reassurance to those that fly them today in a civilian environment.
The corrosion that appears in the extrusions is intergranular corrosion and has the appearance of flaking of the metal. Material that has this corrosion has no structural integrity and can be thought of as a cancer that will spread. Like a cancer patient, once you have it, it can show up in other parts of the body. Once intergranular corrosion is detected in a member, the entire member is suspect. It may be on the surface, but often originates below the surface. If it is below the surface and small enough there will be no visible signs on the surface. The good news is that if it is this small, there is no immediate risk to the aircraft for catastrophic failure. The bad news is that it is there and it will spread with time. The only “cure” for a member with intergranular corrosion is replacement. Like humans, a member with intergranular may live to fly many safe years before it succumbs to being unairworthy.
I have found intergranular corrosion is caused by the composition of the alloy, manufacturing of the alloy, heat treating, and the cycling of temperature, humidity, and stress over time. These factors set the stage for the corrosion with the original processes and original care (or lack thereof) determining the results today. Replacement parts produced today have improved quality control of the alloy and heat treating processes that greatly reduce the susceptibility of extrusions to this corrosion. Add to this a pampered environment and loving care, the replacement longerons of today will have a much longer life than the originals. Properly manufactured longerons today are better than the originals.
Was the longeron defective from the beginning? By the standards at that time, no. Processes were not as well understood and developed as they are today and nobody thought we would be flying them 60 years later.
For those parts of fuselage and wing that are original extrusions, intergranular corrosion is common and typically shows up in a variety of places, the longerons being one such place. The upper longerons are vulnerable where the canopy let moisture in and where it could collect on the longeron. The web just forward of the canopy is one susceptible area and the inboard flange below the canopy is another that seems to deteriorate first. The lower longerons show it where water collects in the H between the front and rear wing attach points and is especially frequent above the radiator. It is best detected by a thorough cleaning (a pressure steam cleaner works well after disassembly). The first thing I would do is a visual inspection of the horizontal web and run my fingers over it feeling for the slightest bulge. The surface may appear fine but you will detect a raised sub-surface blister. Grinding on this will expose the corrosion. It is best if you remove all the paint and apply an aluminum etch. Subsurface corrosion often will show as black areas on the surface. It helps to leave the etch on over night to bring out the black indicator. The black is not a guarantee there is subsurface corrosion, but it is a strong indicator.
Today’s Mustang owner is fortunate that there are a number of shops experienced with these aircraft that provide good professional care using the Structural Repair Manual, Maintenance Manual and Technical Orders as the basis for maintaining them. Areas that deserve special attention for detection of intergranular corrosion are highly loaded extrusions such as the longerons, wing spars and caps and engine mount side members. It is difficult to visually inspect some portions of these, but those hidden areas where moisture could collect are precisely the areas that seem most prone to this problem.
Over the years, I am repeatedly asked many of the same questions and I would like to share my experience and expertise with those grappling with decisions about longerons for P-51s.
1. What if there is only a small area where there is a problem? This one takes some judgment and what is you expectation. From a structural perspective, the aircraft can withstand a lot of deterioration and still be airworthy. Since it could sustain a lot of battle damage and still be strong enough to send back into action, it should be good enough for our civilian lying today with the same damage. Rest easy if you follow the Structural Repair Manual that the plane is sound. Specifically for corrosion, beware as the factors causing the corrosion were present for the entire member, not just the local area where you see it. The entire member had the same heat treat which is a big factor in this problem. Other portions of a member will develop the corrosion over time. If a member has lasted this long, chances are that it had a good heat treat and the corrosion will spread fairly slow, maybe not becoming a problem over our life span. From a structural view point, a member with a little intergranular corrosion probably will be airworthy for a decade or two, maybe longer. This is not the end of this question for most owners today. Most want a better than airworthy aircraft with some wanting pristine planes. For the demanding owner with large financial reserves, replace any member with any corrosion since it is only going to get worse. For most owners it is a judgment call based on the extent of the corrosion.
2. What if one member is bad, what about the others? The heat treating is the key factor and just because the 4 sticks are in the same fuselage does not mean they had the same heat treating. A good stick is still good and nobody knows how long it will stay good; maybe a year, maybe decades. However, all the sticks were subject to the same environment and the chances are that once one goes, the others are on their way – it’s just time. Because of manufacturing, it is more likely the uppers were heat treated alike and the lowers were treated alike. Therefore, if one upper is bad the odds would favor the other upper as the next to go. The lowers could be on a different schedule.
3. What should I do? Simple answer, it depends. If money is no problem and you want the best, as long as it is apart I would replace all four if one shows any corrosion. The cost of the additional three sticks and labor is small compared to the cost to disassemble and you end up with something for the rest of your life. Most owners are more cost conscious and I recommend replacing any member with any sign of intergranular corrosion once the airplane is disassembled. The corrosion is there and probably more extensive than thought, and it is only going to progress. Cheap piece of mind. If the aircraft is still together and not coming apart for other reasons, money is tight, and a very thorough inspection only reveals a little corrosion in one or two isolated places, I would use the Structural Repair Manual as justification for cleaning the corrosion (it is like cancer and will spread so you want to remove as much of it as you can), chemically treating it, paint and have fun flying it but place an inspection watch on this entire member, not just where you found the corrosion. When it shows up again bigger and in more places, it is time to replace.
4. If I replace one longeron, should I replace others? Not necessarily. Back to question 5, if replacing one for any reason, I recommend replacing all longerons that show any intergranular corrosion while it is apart. The marginal cost is relatively small and the planes are very valuable. If only one stick is bad, the decision of whether or not to replace the other side same position (upper to upper, etc) is not clear cut. One argument is that they have the same history and therefore the other is about ready to go; so replace it while you are there. A good decision from a preventive point, but a gamble if it is the best financially. It is perfectly good and you do not really know how long it will stay that way – maybe a very long time which says replacing it is a waste of money. The counter argument is of course that since one went the other is close behind so do it now. My honest advice is that it is truly a gamble and which way do you feel lucky? Most people I talk to make this based on how much money that have and want to spend now. I support either decision as good and valid and even though I sell longerons.
5. For installing my longerons a fuselage jig is not necessary to properly replace longerons if the original fuselage is straight, not distorted and proper locating points. Replace the longerons one at a time using the skins, ribs, back longeron extensions, fuselage wing attach holes and firewall bolted to the “bath tub” fittings as locating points. The “bath tub” fittings are critical and utmost care is necessary in their installation. Make sure the holes are exactly aligned and that the fasteners have an interference fit in each hole. This is necessary as the edges of the holes are placed in compression this way and less likely to fatigue. High Shears work well and can be obtained in oversize diameters. Try to avoid going to the next larger nominal size however. The tails of the main longerons are vulnerable to insufficient hole edge distance which must be closely scrutinized.
In summary, if the fuselage is separated with a longeron having any intergranular corrosion or there is significant corrosion in a longeron in a flying aircraft, I recommend replacing it within a year (provided it is airworthy based on the Structural Repair Manual). If money is not a problem and you want a top aircraft, replace all four at the same time. What to do in between these guidelines is a judgment call of the owner and any decision is a good one.