What is ACM Anyway?

A Fighter Pilot’s trek down the fatigue-life highway.
 

By LTC David Rothenanger

Fighter pilots can be an arrogant, cocky lot with planet-sized egos. But who wants a wimpy, milquetoast kind of guy fighting your wars? Fighting a dogfight is all about winning. NOTHING else matters. “There are no points for second place!” “If you ain’t cheating you ain’t trying!” A dogfight is the ultimate manly sport. The only thing that comes even remotely close is jousting! Today, the F-16 has the ability to accelerate in afterburner to over 450 knots in the length of the runway and climb straight up through 20,000 feet. This fighter jet provides a ride that few in aviation circles would turn down if given the chance. Go to any big-city air show and you can find “Tom Cruise” standing by his jet in his flight suit and sunglasses. The crowd of spectators pound him with comments as to how cool it must be to be a real fighter pilot. He gives them all an approving nod.

Enter the commercial ACM operators: “Yes, you too, can be a fighter pilot” “Just sign up for our Fighter Pilot for a Day Program and we will put you in a real air-to-air engagement in one of our T-34s”. “You fly the airplane yourself, wear a real flight suit and helmet, and even get a call sign and videotape.” “But more importantly, you get a chance to win this contest and prove your manhood by kicking your coworkers butt in a real dogfight.” “I bet you can just taste the fun.”

It is this very attitude coupled with a lack of flight discipline and real world physics that has lead to three fatal in-flight structural failures in T-34s used in commercial simulated air combat. At least two other non-fatal incidents occurred where their T-34s sustained major structural damage and underwent extensive repairs. If you consider only 10 or so T-34 airframes have been involved in commercial combat simulations, you can see that they have had a horrific safety record. The military has not been immune to these same three problems. They have put great effort into developing effective programs for fatigue-life monitoring that yields equipment and a training methodology that is effective and as safe as possible. The real fighter pilot world has had many failures in these areas that resulted in both the loss of life and aircraft. It is my intention to point out, from a real fighter pilot’s perspective, how the ACM operators marched down the very same path. Their operation, and more specifically, fatigue-life accumulation, is vastly different from the majority of other T-34 operations that are in private, non-commercial usage.

What is ACM anyway? Air Force Manual 3-1, the fighter pilot’s bible, is a source of terminology and tactics that forms the basis for the way we employ our jets in air-to-air combat. In layman’s terms, ACM is Air Combat Maneuvering. This term can be loosely applied when describing any air-to-air activities. However, the classic definition involves advanced maneuvering with three aircraft in a 2-v-1 (2 versus 1) scenario. The commercial operators have chosen the term “ACM” (I guess because it sounds cool) to describe what they do. However, they usually engage in a 1-v-1 (one versus one) scenario. This is known as BFM, or basic fighter maneuvering. They may at times have three aircraft airborne, but usually “engage” only two aircraft at a time when flying with customers. BFM is practiced from many different starting points, or setups, but the most common is the ‘perch’ setup. In this scenario, one aircraft (the offensive fighter) starts the engagement from a position of advantage behind the other aircraft (the defensive fighter), usually about 30˚ off his tail and some set distance back. The offensive fighter’s objective is to maneuver his aircraft into a ‘weapon employment zone’ (WEZ), turn on the gun, and “kill” the ‘Bandit’ while the defensive bandit’s objective is to survive by flying in an unpredictable manner in an attempt to defeat the fighter’s attack. Another more advanced setup, called the “butterfly” or “neutral” set up, is used for full up maneuvering where both players start from a neutral head on pass. Neither one starts with an advantage. Both pilots objective is to “kill” the other guy. Only one pilot can “win” this aerial duel.

To understand the stresses imposed on an aircraft during BFM, one must understand the radically changing dynamics involved. In an entry-level setup, you start with a “canned” or predictable Bandit. From the initial perch set up, both aircraft call “Ready” and then a “Fight’s On” call is made. The defensive Bandit enters at a level 3 or 4G turn, stays in that “canned” turn until the offensive fighter maneuvers just behind the Bandit’s tail, takes a gunshot, and calls a “kill”. The maneuvering required to get the kill is called BFM. The offensive fighter must control his speed, overtake, and angles in order to solve the problem presented by the Bandit. A novice tends to use “brute force” (i.e. more Gs than the Bandit), while an experienced pilot will be able to “BFM” his way to a solution using vertical maneuvering. This dynamic ever changing maneuvering in three dimensions uses elements of acrobatic flight, including parts of loops, barrel rolls, and vertical pulls.

The maneuvering quickly gets very aggressive when we raise the bar, turn the Bandit loose and let him maneuver freely without restriction. The Gs required to solve the BFM problem increases dramatically as the Bandit pulls harder. The conditions are now ripe for an “over-G” situation, especially with asymmetric rolling Gs. A rolling or asymmetric G-loading is defined by a loaded pull, coupled with any roll rate. A typical rolling limit on many aircraft is 2/3 of the maximum G. The T-34 has a 4G rolling limit. It is very easy to over G an airplane when engaged in a dogfight, especially while rolling. It takes skill and great feel for the aircraft to maintain G-forces within limits. In a symmetrical pull, a balanced load is placed on both wings. The angle of attack increases as you pull back on the pole and demand more Gs. If you roll the aircraft during this pull, one wing has to produce more lift than the other does in order to generate the roll. This unbalanced, asymmetrical loading, typically found during BFM, is very damaging in terms of fatigue-life usage. The finite data analysis performed on the T-34 wing structure by the T-34 Spar Corporation fully supports this increased loading with solid, empirical facts.

Unrestricted engagements that begin from a neutral starting point can last 5 minutes or longer. They will include numerous aggressive high-G cycles. In a typical one-hour sortie, it would be very easy to accomplish five full up engagements and accumulate a hundred g-cycles. This is way outside the design spectrum of a training aircraft even if the Gs are held within limits. If you over-G the aircraft in this process, all bets are off in terms of fatigue life accumulation.

I witnessed the damaging effects of excessive of G cycles and over Gs first hand as an instructor pilot in the AT-38B while stationed at Holloman Air Force during the 1980s. In an effort to save training costs, the U.S. Air Force developed what is known as Lead-In Fighter Training or “LIFT” as it is commonly called. A standard T-38 was modified into a ‘beginner’ fighter jet, complete with an external bomb pod and gun sight. This AT-38B was more economical to operate than the F-15s and F-16s that were being used for BFM training, so it became the new equipment of choice for basic fighter training. We flew hundreds and hundreds of BFM missions with thousands of perch setups while training our newest fighter pilots. It was not long before we started developing serious fatigue cracking problems in the wings. We experienced several in-flight structural failures where we lost a wing tip during an engagement. The jets were able to land safely, but they were missing nearly 3 feet of wingtip! We realized we had a big problem. The constant stress of numerous G-cycles per hour and many over Gs had finally taken its toll.

Even though it flew its fair share of acrobatic maneuvers in primary training in the Air Force, the T-38 never experienced any of the same structural or fatigue issues that the AT-38s were facing. In short, the AT-38s were being “BFM-ed” to death! The Air Force engineers determined that our excessive G-cycles per hour and regular asymmetrical over-Gs had dramatically consumed the fatigue life of our AT-38s. The root cause of our problem was pure and simple abuse. We learned some major lessons, especially in how we applied the G-loading during BFM training. We stopped doing loaded rolls and put the emphasis on lift-vector control. We would simply pull in one plane of motion, unload, roll to the desired direction, then re-apply the G. We made it plain to everyone that an occasional over-G was no big deal as long as we ended the flight, came home, and reported the over-G to maintenance so the aircraft could be inspected. In addition to modifying our flying behavior, we also performed several fatigue-life enhancements, including putting thicker skin on the wings and beefing up several critical structural areas.

The F-16 also has had its share of fatigue-life issues over the years as it struggles to meet the demands of a high operational tempo. The F-16 was originally designed to be a low-cost, daytime VFR fighter. Over time, we have added mission after mission to our playbook. We increased the gross weight as more equipment was added to the jet. The F-16 is a 9G jet that is flown at a very high G and high G-cycle rate per hour. It would have never met its projected service life had it not been for several fatigue-life and structural enhancement programs. When computing fatigue-life accumulation, the F-16 community uses an equivalent hour program based on types of use. A complete air-to-air mission counts much more than a cross-country flight. Because of this use, the F-16 is inspected on a regular basis, especially after an over-G. The C-130 fleet was recently placed on an equivalent hour program based on use. Many of the C-130’s with as many as 45,000 airframe hours were grounded because of fatigue issues caused by recent hi-demand combat operations.

Putting physics aside for a moment, there are several other factors that affect the fatigue life accumulation on an aircraft. One of these factors is the attitude of the pilot. Many pilots possess an “I must win no matter what” attitude, which often leads to a lack of flight discipline. Fighter pilots view an air-to-air dogfight as the ULTIMATE CONTEST. The problem with this is that sometimes rules and limits are exceeded in an all out competitive attempt to win. The Air Force has developed several rules of engagement (ROEs) that govern its fighter pilots when engaged in dogfights. Although fighter pilots consider it cool and manly to pull Gs, one such ROE states that the flight must be terminated with a “knock it off” if an over-G occurs.

There have been several classic accidents over the years where two adrenalin-charged fighter pilots BFM-ed himself into a mid-air collision. In the course of this “contest”, several ROEs were violated in an attempt to win. Pilots have needlessly died and jets were lost in the process. The senior Air Force leadership put an end to this by demanding that pilots change their philosophy regarding engagement; no diametrically opposed objectives! The new thinking is that there are no winners or losers in BFM contests. Instead, one pilot is designated the “Training Aid” and he limits his use of the afterburner, pulls limited Gs, or purposely makes common BFM errors. Since one pilot is little more than a flying platform for the other’s benefit, it is no longer a contest between two Fighter Pilots who think that they “must win”. This new way of doing “business” serves to reign in the egos and makes the entire process safer overall. Apparently, it is okay to “lose” when you are the Training Aid.

Moving back into the world of commercial T-34 ACM operators, one can clearly see numerous examples of the same ego-driven attitudes, lack of basic flight discipline, and disregard for the laws of physics mentioned above. If one closely examines the video tapes from these engagements, the typical pattern of abuse quickly emerges. In several places on these tapes one can hear the official “fighter pilot” muttering and cussing under his breath because he is losing ground in the contest or “engagement”; he is getting his butt kicked. He knows that losing the BFM contest is unacceptable. The offensive guy, on the other hand, is rolling and pulling as hard as he can. He can be heard saying, “Just a little more turn and he’s mine.” Although the video does not include a shot of the G-meter or airspeed indicator, you can still get a good idea of what is going on by looking at the nose-tracking rate across the clouds or ground. The setups usually started around 175 knots, so when you see a hard turn to the buffet or accelerated stall point, (where the nose track rate slows or stops), you know they just pulled close or exceeded the maximum available G. The V/G diagram for the T-34 shows that if you are at maneuvering speed (152 knots), and you pull to the buffet, you just pulled to the 6G line. If you accelerate to 180 knots, you have almost 9Gs available. In addition, over-pulling at high G to the buffet compounds the stress because wing buffet is also a G-cycle stress. Roll rate is easy to see and after viewing a few of these video tapes, one gets a good feel for the nose rate/current-G relationship. We record and review every BFM mission that is flown in the F-16. I personally have reviewed hundreds of hours of air-to-air footage over the last 25 years, so I am very familiar with BFM video review and analysis. The VCR tapes from the ACM operators documents numerous over-Gs. The “I Must Win” attitude and total lack of flight discipline becomes very apparent, especially when you hear comments like, “WOW, we just pulled over 6Gs” and “Hey, you better not say that over the radio” and “We’re going to need a wheelbarrow to pick up the pieces”. They even laugh and seem proud that they pulled so hard! On many missions, the instructor would do a demonstration of a full-up neutral engagement if their customers were up to it. I have reviewed tapes where the instructor called for “Cameras Off” prior to the demo engagement. I can only imagine what went on with the cameras off.
A few years ago, at my job with United Airlines, I flew with a former fighter pilot who was a part time instructor pilot with one of the ACM operators. He told me that he was “paid” in flying hours in the T-34. He went on to say that he used to get together with the other instructors so that they could spend their ‘pay’ in a full up, “let’s take off the gloves” dogfight. They did NOT film these flights. Gosh, I wonder why? I asked him about G-limits and he was unaware that the T-34 even had a rolling G-limit.

Most of the commercial ACM operators hire former or retired fighter pilots as their instructors. These are the guys who knew the most about BFM and how to set up simulated “Fighter Pilot” programs. They know how to build mock fighter squadron buildings, complete with pictures of real fighter jets, and inspirational banners such as “Through these doors pass the World’s Greatest Fighter Pilots” and “Fly, Fight, Win!” The long-retired military T-34 trainer seemed the perfect choice for the mission. With its military markings, camouflage paint scheme, bubble canopy and stick, it looked just like a mini-fighter plane. It was relatively easy to fly and economical to operate. However, there was one major problem. It was NEVER designed to be a BFM machine. The T-34, just like the T-38, is not a fighter; it is a trainer. It was designed and built to be flown by the military as a primary trainer. Primary military programs include transition training (take-offs and landings), instrument work, formation flying, cross-country navigation, and aerobatics. The aerobatic part of the training made up only a small portion of the program in terms of total number of hours flown. These hours included mostly symmetrical maneuvers such as loops and rolls. The commercial ACM outfits decided to adopt the T-34 as their new ‘fighter’ plane anyway. The instructors eagerly looked forward to getting back into the BFM game. They had no idea how much fatigue-life they would gobble up as they eagerly took the T-34 into the air-to-air arena.

The G-limits for the T-34 are based on operating the aircraft within the gross weight limit. The commercial ACM operators added cameras, VCRs, gun sights, laser sensors, and smoke systems to their T-34s. The weight of this equipment, combined with the extra weight of required parachutes would push the gross weight limit of the aircraft if the mission were flown with full fuel tanks. I do not think it was company policy to weigh the instructors and their customers, so if both persons on board happened to be on the large size, it could easily be over gross weight. I weigh 215 pounds and if I top off the fuel tanks of my T-34B, then add the weight of my two parachutes, I only have about 120 pounds left for a passenger. If I want to carry a bigger person, I must download fuel. I am willing to bet that many of the missions accomplished by ACM operators were flown over gross weight. I have a copy of the U.S. Department of Interior operational memorandum (#05-50) dated January 1, 2005. This memo gives the department the operating guidelines and authority to fly their Amphibious Cessna 206 over gross-weight. They are required to do additional inspections and install a continuous electronic accelerometer to monitor airframe stress. Sadly, the ACM operators had no such monitoring program in place and did not do any additional inspections.

After the first T-34 wing separation in 1999 and subsequent investigation, the National Transportation Safety Board made the following recommendations:

---"The Safety Board believes that the FAA should require the development of operational and inspection criteria for the critical flight control surfaces and structural elements, as necessary, for any airplane used in air combat simulation flights."

It is obvious that the NTSB determined that there was a difference in how the ACM operators were using their T-34s, as they went on to state:

----"The Safety Board has found no other examples of possible wing structure fatigue cracking in the long history of the T-34 series of airplanes. Although typical wing loading spectra generated by training missions, aerobatics, and air combat simulation flights are not available, the Safety Board believes that air combat simulation flights may induce a larger number of high positive wing loading events per flight hour than other operations. In addition, the multiple initiation sites and large number of fatigue cracks in the wing structure suggest that the fatigue damage is both recent and related to high stress. Considering these factors, it is likely that the fatigue cracking on the accident airplane was the result of the loading conditions experienced during the air combat simulation flights.'

---"Beech YT-34, T-34A, and T-34B airplanes operated only for personal and aerobatic use are of somewhat lesser concern than those used for air combat simulation flights. Although these airplanes, especially those regularly performing aerobatics, may experience occasional G loads of the same magnitude as those experienced by air combat simulation airplanes, the number of high G load applications would be expected to be much less for the airplanes in personal and aerobatic use."

Some may argue that prior to the first T-34 accident, by the ACM operators were unaware of the amount and extent of damage that was occurring in their aircraft. However, afterwards, they should have known better. If they had followed the NTSB recommendations and FAA restrictions, the second and third accidents could have been prevented. Yet, it gets worse. After the second accident, the NTSB determined that one of the operators had totally disregarded the flying restrictions imposed after the first accident and had over-flown the specified grace period for implementing said restrictions. This total and wanton disregard for the rules is inexcusable. They continued to conduct upset training (recovery from unusual attitudes, which is an exercise ripe with potential for over Gs) in their T-34s during this period of flight restrictions. The T-34s had been restricted by FAA regulation to no aerobatic flight, 2½ Gs, and a maximum speed of 152 knots yet, amazingly, this outfit had a picture of one of their T-34s upside down on the cover on the cover of the April 2001 issue of FLYING magazine!

I am convinced that the center section of the T-34 involved in the third accident had pre-existing fatigue cracking from previous hours of abuse. Although its wings had been repaired with Baron Spars, it still suffered a structural failure.

It is vitally important that we maintain and fly our prized T-34s with Safety as the Number One Priority. We all want this. However, when developing fatigue-life timelines, rules, inspections, etc. we need to apply logic and common sense. We want and need to verify, monitor and maintain the structural integrity of our T-34 fleet. But, we want to do it fairly. We do not want to be unfairly restricted or denied the privilege to fly based on the irresponsible actions of a few. The first step in this process is to place each aircraft appropriately on a logically established fatigue life highway that is based on aircraft condition, type and intensity of use, and number of G-cycles per hour. If you go to Oshkosh or Sun-n-Fun, you will be amazed at the number of quality, pristine T-34s that you find on the ramp. Over the last decade, several of these T-34s were among the top award winners. Numerous owners have spared no expense restoring their prized T-34s to perfection. The EAA Warbirds of America have recognized several of these fine examples of craftsmanship with their top “Grand Champion” and “Reserve Grand Champion” awards. These aircraft are not just “paint jobs” (“a pig in a tux”) many have been stripped to the bone, carefully inspected and had numerous structural parts, flight controls and hardware replaced with new parts. Yes, the T-34 may an older aircraft but most owners maintain and document their aircraft at a level that is well above the typical single engine light airplane that you find sitting on the ramp at your local airport.

The restoration process accomplished by many T-34 owners, completion of the approved AMOCS, which included cold working critical structural areas, and replacing or rebuilding the spars, has served to preserve, and even extend our available fatigue-life. The inspections performed thus far have served to verify and confirm the structural integrity of our fleet. Over the course of the last few years, we have eddy-current inspected almost 6000 rivet holes in the T-34 fleet. Only one revealed any cracking. The crack found, was (you guessed it) in one of the commercial ACM operator’s T-34s! We inspected the entire flying T-34 fleet at every location where fatigue cracking was identified in the first two accident aircraft. We found more or less nothing. There was not a global fatigue problem. When the inspection process is developed for the center section (failure point in the third accident), I believe we will find the same.

Condition, type of use, and intensity of use are the driving factors that should establish the rightful position of each T-34 on the fatigue life highway. The publication “Best Practices Guide for Maintaining Aging General Aviation Airplanes” dated September 2003, which is endorsed by the FAA, discusses condition and usage as key factors in aircraft aging. The T-34 enjoyed a very long 50-year history of safe structural integrity prior to the three in-flight failures experienced by the commercial ACM operators. They were clearly flying beyond the design limits of their aircraft. The NTSB, in their accident investigation report from the first accident, stated the probable cause as; “The safety pilot exceeded the airframe design limits that resulted in the overload failure of the right wing.” I am tired of reading article after article in various aviation publications about the “aging” and “wing spar” issues that are accruing in the T-34 fleet. If you honestly look at the facts surrounding these three accidents, it is clearly NOT a fleet-wide T-34 aging-aircraft issue. It is pure and simple operator error caused by a lack of integrity, poor flight discipline, poor supervision, poor maintenance, and a total disregard for the stress that their intense, high-G operation was placing on their aircraft. The NTSB reported that the ACM operator involved in the first accident had accumulated over 8400 simulated combat missions in their fleet of three T-34s. The accident aircraft had accumulated 8200 total hours of which roughly 4000 were accumulated in the ACM fleet. This accumulation is actually, in my opinion, a testament to the robust nature of the T-34 structure as it survived thousands of hours of abuse prior to failure.

Once we develop an inspection for the center section and get back into the air, we must develop a system to document our type and intensity of use so we can verify our march down the fatigue life highway. Future inspections and the hour accumulation allowed should be driven by the intensity and type of use. Using the abusive hours accumulated by the ACM fleet in calculation to determine failure points under normal operations is scientifically and statistically unsound. We need to continue to educate ourselves on safety, accident prevention, flight discipline, and how fatigue accumulates on our aircraft. I am all for installing a recording G-meter in my T-34 so I can download an annual history of G-cycles and protect myself from future grounding. I will show everyone that I am sipping my fatigue life through the thinnest of straws.

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(LTC Rothenanger has been flying fighters for the military for over 25 years. He is currently an F-16 pilot for the Indiana Air National Guard and a 767 pilot for United Airlines. He has a degree in aerospace engineering is a certified A&P mechanic and is the owner, operator & maintainer of an award-winning, pristine T-34B)