SR-71 Maintenance

Posted on: 15 January 1997
Last updated on: 15 January 1997

SR-71 MAINTENANCE (July 1984 - December 1989):

Let's start with a brief tour of the aircraft.

When I arrived, I was most interested to see the cockpit of the SR, and knew it had to be an assortment of high tech. instrumentation and screens. What I saw was not even close, and amazed me. Although the Forward Cockpit (FCP) was filled with more instrumentation than other Air Force (AF) aircraft, it was the same old 60's technology. Actually, it was quite similar to that found in the old "bent-wing" F-4s. The Aft Cockpit (ACP) is a bit better, with mostly navigation and sensor control panels. No Star Wars technology here, as all the really neat stuff lies inside the many mission bays, out of view from prying eyes. Both cockpits are very spacious compared to that of fighters, and are equipped with ejection seats. The engine throttles are located on the left console, and tons of circuit breakers line both consoles. Only the front seater has a control stick to fly the aircraft. Map projectors were installed before flight in both cockpits. These were just scrolling imagery films, put together so that the entire mission route could be seen and tracked by both crew members. An optical view sight was installed in the ACP to allow the Reconnaissance System Officer (RSO) to view the target areas. Later, a video viewsight replaced the ancient optical one. Both the map projectors and view sights are not incorporated in todays SR-71s. Later on, a Peripheral Vision Display (PVD) was installed in the FCP. It is a laser, that projects a horizon line across the entire instrument panel, assisting the Pilot in flying. It also makes for a great light show on those winter evenings when the California fog rolls into the hangers. One cute little device found in the cockpits is what we call a "dingy stabber". This is just a six inch long flat piece of metal with a pointed end, and used for puncturing life rafts should they inflate accidentally in flight. For some reason, these became highly sought souvenirs during the program closure.

The airframe is primarily titanium alloy, covered by metal skin with plastic (honeycomb construction with asbestos) and metal panels. Many louvered panels surround each nacelle which allow for inlet inflows and outflows. The entire fuselage aft of the cockpits are fuel tanks. They number one through five from front to back. The inboard wings also house fuel cells, which incorporate part of tank three and two sections of tank six. The outboard wings are hinged near the top of the nacelle, and fold inward to allow access to the Pratt & Whitney J-58 engines. All electronic, communication, and mission bays are located on both sides of the lower fuselage beneath chine panels, and start just in front of the FCP extending aft to the 715 splitline, which is where the wings start. There is one other bay in the forward section of the Nose Landing Gear (NLG) wheel well which houses more circuit breakers, and is currently used to house the data-link system. The nose section is changeable according to each mission. Atop the aircraft behind the ACP and above tank one is the Astro Navigation System (ANS). This is the avionics package that navigates the SR-71 by the position of stars, weather day or night. We do have a hard time getting it to operate inside the hanger though. A mission tape is also loaded into this system, and is interfaced with the Auto Pilot system and various sensors for automatic operation. Just aft of the ANS package is the In-Flight Refueling (IFR) receptacle. Also atop just aft of the Main Landing Gear (MLG) wheel wells is the drag chute compartment. All landing gear struts are titanium, and nitrogen filled. There are two tires on the NLG, and three on each MLG.

Many internal systems are worth mentioning. There are three ten liter Liquid Oxygen (LOX) storage converters located under the left chine panels below the FCP. System one supplies the FCP, two supplies the ACP, and the standby system supplies both. Of course, these systems are used to sustain aircrew breathing at high altitude. Three Liquid Nitrogen (LN2) storage dewers are required to pressurize the fuel system. Two are located in the aft section of the NLG wheel well, with the other under the left chine panels just forward of the LOX system. Four hydraulic systems (L, A, B, and R) are incorporated, with engine driven pumps. The fluid was specifically designed for the high temperatures of the SR-71 and is clear in color. L and A reservoirs are located in the left MLG wheel well, with B and R in the right. L and R are used to control the inlet systems, NLG steering, MLG brakes, and the IFR receptacle. A and B are used to control all flight control surfaces, and assist in NLG steering through rudder movement. R is a backup system for L, with B a backup for A. The primary and backup systems are driven by different engines to facilitate safe flight in the event of an engine failure. The heart of the aircraft's electrical system is the "load center" located in the electronics bay beneath the chine panels on the left side. It is powered by ground equipment, or in-flight by a pair of engine driven generators, Constant Speed Drives (CSD), and an Accessory Drive System (ADS). Due to the many sensors and systems operated, the SR-71 electrical system can adequately power a small city. JP-7 is the fuel used, and is specifically designed for high altitude flight and high temperatures. All fuel system tanks are filled to a specific amount for each mission profile. In flight, they are usually filled to capacity, and an auto-sequencing system controls the flow to ensure a proper Center of Gravity (CG). Fuel is also re-circulated through the system to be cooled, and also provides some cooling to the engines and generators. Two engine driven oil systems are used to lubricate the J-58s, and are located on each engine. This oil is also clear in color, and was specifically designed for the high temperatures encountered. This oil must be heated to seventy degrees Fahrenheit prior to each engine start. Below this temperature, it is as thick as molasses. The environmental systems "heat-sinc packages" are located in the lower inboard section of both nacelles. They are fuel cooled, and provide engine bleed air to the air-conditioning package located in the ANS compartment. This package primarily provides cooled air for the pressurization and cooling of the cockpits, nose, mission bays, and ANS bay. Triethylborane (TEB) is used in place of igniter plugs to start each engine due to the high flash point of JP-7. TEB is a volatile fluid that ignites when it comes in contact with air. It is serviced prior to each flight and is stored in a sealed tank mounted atop each engine. With engine rotation, TEB is released by placing the throttle over the hump to the idle position, then again when placed into the After Burner (AB) position.

The inlet system is the heart and soul of mach three flight, and is controlled by many factors. There are three Digital Automated Flight Control System (DAFCS) computers, A, B and M, and are also located in the electronics bay. There are three, a primary and two backups, because the inlet system is critical for aircraft safety at high speeds. These computers receive there inputs from the alpha and beta probes on the pitot tube through the Pressure Transducer Assembly (PTA) located just aft of the nose assembly. The computers then send a signal to the Differential Pressure Transducers (DPRTs or "hot boxes") located on both lower inboard nacelles. These units send signals to the flight control servos, and actuate the inlet control systems, such as the spikes and forward and aft bypass doors. The spikes are the pointed conical sections which extend from the front of the inlet of each nacelle. These spikes can travel as much as twenty-one inches aft, and are used to position the super-sonic shock wave inside the inlet. Combined with the back pressure from the face of the engine, the supersonic airflow enters the forward bypass doors. This ensures that supersonic air will not enter the engine, with some being bled overboard, and the rest bypassed around the engine, and dumped into the aft engine sections for greater thrust. The aft bypass doors perform similar functions. In fact, about 75% of the actual thrust is generated by this bypassing air, and allows the engine to operate economically, not using much more fuel at cruise speeds than it does on the ground.

You've probably heard the SR-71 is a severe leaker, and I'll try to put this into perspective. Once LN2 is serviced a few hours prior to launch, the fuel system becomes pressurized, and that's when the real leaks start. Normally, about five or six steady fuel leaks (about the width of a drinking straw) show up coming from both inboard wings, falling about six feet to the ground. The entire bottom of the fuselage becomes wet, and starts dripping onto the hanger floor. Some puddling starts to accumulate on top of the inboard wings, and at times runs off the wing onto the floor. In some bad leakers, fountains can be seen spraying upward from the top of the inboard wings, ranging anywhere from two inches to three feet in height. Usually, the really bad leaks occur when the aircraft is getting close to being sent to the Depot for an overhaul. How much fuel is actually lost prior to flight? It was a common practice to refuel the aircraft about four or five hours prior to flight. It was also standard to place about four to seven hundred pounds of JP-7 extra in the tanks to allow for this leakage. That's a loss of about one hundred pounds or sixteen gallons per hour. And folks, that's just for a standard fuel load. At times, due to lack of tankers, we would put considerably more fuel onboard, and launch her on a "rocket ride". When we did this, you could basically double the amount of leaks I've described. Why all the leaks? High temperature fuel sealant was especially designed for the SR-71, and there's no other substance known in existence to replace it. Once the aircraft is as cruise speeds, it tends to seal itself. The leaks I've spoken of do not jeopardize the safety of the aircraft, due to the high flash point of JP-7. In fact, a lit match thrown into it would just go out. Up until the late 80's, the fuel leaked was simply washed out of the hanger after the launch, and went into the ground. Due to environmental laws towards the end of the program, we started to catch the fuel in drip pans, dispose of it properly, and vacuum the residual from the floor. You could always pick out the guys who had participated in a launch. They smelled like JP-7, there hair was sticky looking, and fuel stains covered their uniforms. Many guys wore rain suits to eliminate this problem. Believe it or not, a half can of Coke added to the wash removed all the stains and smells from the clothing.

We had two methods to start the powerful J-58 engines. The first and oldest method was to use the start carts. These were twin buick 455 v-8 engines, dressed out with racing cams and headers. They were coupled together to a rotating shaft that connected to the starter shaft of the engine. It took some serious rpms to start the J-58s, and it wasn't uncommon to see one break apart throwing piston rods through the engine block. In the late 80s, the buick engines were replaced by built up Chevy 454s. That pretty much eliminated our rod throwing problem. The other method to start the engines was through the use of pneumatic air. The hangers at BAFB were equipped with these systems in the early 80s, and it quickly became the preferred method. A removable turbine was attached to the engine starter shaft, and was fed by two four inch diameter hoses connected to the system on the hanger wall. The Crew Chief operated the valve from the wall location. We would still use the start carts periodically to keep personnel current, as we had to use them on the trim pad, and when at Temporary Duty (TDY) locations at times. We did use the pneumatic system off station at times, but this was quite cumbersome. It required four ground air units (-60s) to start a single engine. These four units are manifolded together, then connected to the two entry points on the starter turbines. This is the method now currently used by Detachment (Det) 2 at Edwards Air Force Base (EAFB).

Performing engine runs was the best part of the job. To perform this task, an engine run school had to be accomplished, and was limited to those at the rank of Staff Sergeant (SSgt) and above. There's nothing like the feeling of true power you get just sitting in that front seat with engines running. We were permitted to perform engine runs slightly below Military (MIL) power inside the hangers, and could also accomplish MIL runs on the ramp next to the shelters. Usually, the jet engine mechanics performed the trim runs in full AB, but at times, even us lowly Crew Chiefs got our chance. To get to the trim pad at BAFB, it was about a mile and a half tow south down the taxiway from the hangers. When a trim run is performed, the aircraft is held in place by huge steel bars connected from the nacelles to a ground attach point well behind the aircraft. The NLG is also anchored by cables. The SR-71 is probably the easiest aircraft in the world to start. Once engine rotation has begun by the ground starting unit, you simply move the throttle to the idle position and watch the gages. Rpm increases, TEB lights the fuel, oil, hydraulic and fuel flow pressures build up, the engine starts and Exhaust Gas Temperatures (EGT) begin to climb, all stabilize, and thats it! Different from other AF aircraft are the engine speed gage ( which is measured by rpm, not percentage), and the fuel quantity gauge (measured in pounds, not gallons). Once you're running, it takes about thirty minutes for the entire trim procedure, and we're kept quite busy recording numerous readings. There are more checks during this engine run than on any other aircraft I've encountered. When you're sitting there in idle, you can clearly see the crew entrance stand off your left side. When you take her to the MIL power setting, the nose of the aircraft drives down, and the stand appears to have raised two feet. When you take her to minimum AB, the nose drives down even further, and the stand all but disappears from view. Once in AB, the aircraft feels like it flying as it moves rapidly from side to side. When you nudge her to maximum AB, you can barely hear through the communications gear worn for contact with ground personnel, and the flame coming from the back grows from five to about twenty feet. Once it's all over, you slowly bring her back to MIL, then idle, and the nose comes back up, and it seems quite calm. After giving the J-58s a five minute cool down period, you shut them down.

TEB servicing was quite interesting. Early on, there was a special TEB shop to perform this task. Then in the late 80s, the Crew Chiefs took over this function. Any time this operation was performed, the Fire Department would be on the scene in case of trouble. Two mechanics were required, and both wore complete fire suits. One manned the TEB servicing cart on the ground, with the other atop the wings at the servicing point. Once servicing was under way, the mechanic on the wing carefully monitored the process for leaks. Leaks were frequent, and usually quickly corrected. At times, larger leaks occurred, and the mechanic would simply take a wet rag and rap it around the leak to smother it until the process was complete. I've seen leaks so bad, that the mechanic would pull his gloved hand from within the servicing panel, and it would be completely engulfed in flames. He would simply put his glove into a bucket of water, cool it off a bit, then press on. Sometimes the TEB servicing cart would also have small fires, and they were also quickly fixed by wrapping a wet rag around them. The BAFB Fire Department was usually well trained on how to deal with these fires. We always briefed them before we started, and they knew they should not respond unless we instructed them to. Well, sometimes you get a new guy, and on one such occasion, he got a bit anxious of a small fire, and unleashed a spray of water towards the wing and the mechanic performing the task. Unfortunately, the mechanic was blown off the wing, and broke his arm. The other mechanic quickly took care of the fire. There were rare occasions when the fire would drop on to the wing. In those cases, the Fire Department would spray it off the wing, and we'd let it burn itself out. Even if you kept it wet, once the hose was turned off, the fire would start again. It was also interesting to watch the TEB shop perform standard maintenance on the servicing cart. When lines were disconnected to change the filters, the filter elements would be extracted on fire. As you can imagine, every time we went off station with the SR-71, the local Fire Departments usually freaked out a bit when we told them of the operation we needed to perform.

Refueling the aircraft is a bit archaic, and four mechanics are required. One is the ground supervisor, one the fire guard, one monitors the fuel source, and the other performs the operation from the FCP. We usually refueled from ground servicing hydrants, and this was connected to our Single Point Refueling (SPR) receptacle on the right side of the fuselage just aft of the NLG. The mechanic in the FCP controlled the refuel box which was connected to a port in the NLG wheel well. This box controls the primary and secondary refuel shutoff valves in each tank. He would monitor the fuel tank quantities on the instrument panel, and turn off each tank as it neared the desired level. Many times, some fuel would go where it wasnít supposed to, and when that happened we would simply transfer the fuel between tanks. This could be accomplished by two methods. Without extra equipment, we could transfer fuel forward into tank one or aft into tank five. We accomplished this by switching on either the aft or forward fuel transfer switch on the FCP instrument panel, and applying boost pump pressure to the tank that fuel would be removed from. When fuel was needed in any other tanks, we would hook up what we called a "run-around" hose between the SPR receptacle and the defuel receptacle, which was located on the same side near the 715 splitline area. Then, using boost pump pressure and the refuel box, fuel was transferred. Defueling was easily accomplished by boost pump pressure through the defuel receptacle. Weather refueling or defueling, it was a real pain to remove all the fuel from tank six. To accomplish this, the MLG struts had to be deflated, and the NLG strut fully extended. This gave us a nose up attitude, and allowed the fuel in tank six to reach the boost pump area. Since JP-7 has a high flash point, we were able to perform maintenance on the aircraft during fueling operations, which is usually not permitted on other AF aircraft. There was one very real safety concern to watch for during fueling operations. Sometimes a small amount of TEB would seep into one of the engines. This didn't happen often, but when it did, you definitely knew it. TEB has a distinctive odor, and that was the first sign. Next, smoke would appear coming from the affected nacelle. Usually it would burn itself out, and all was well. If it persisted, we would have to motor the engine to either blow it out, or eject it out the back of the engine. This only happened to me five or six times, and I only had to motor the engine twice. I found it quite interesting that every time it did happen, the fuel mechanic monitoring the hydrant would quickly disappear.

Let me say a little bit about the work force. The experience level and pride of our mechanics was without a doubt the best I had ever seen in the AF. This was due to the love for the aircraft, and a program called "code 42". Once a mechanic had been on station for a couple of years, he could apply for this code, which would keep him in the program for at least five additional years. Most mechanics I knew did exactly that, and in fact, it wasn't too uncommon to see folks hang around for more than ten years. Some spent their whole career on the SR-71. This made us very rank heavy, and very few new trainees were brought in. You've always got a few bad apples, but for the most part, everyone knew what they were doing. Integrity was extremely high, and very few mishaps occurred. This was one big family folks! Early on, we Crew Chief types worked a three shift operation, and each aircraft had it's own head Crew Chief. The rest of the personnel were in a floating pool and were assigned daily. There were also two "BPO/Lube" teams which took care of all after flight inspections and periodic lubrications. These functions will be discussed later. Towards the late 80s, we changed our maintenance concept. All Crew Chief mechanics were divided evenly and assigned to a specific aircraft. From then on, each aircraft had an Aircraft Manager (a fancy name for Crew Chief) and about ten mechanics permanently assigned. This worked out extremely well, and we basically worked our shifts around the missions. The only time we did not work on our own aircraft was when others needed help, and of course about once every six weeks on a rotating weekend duty schedule. Almost without exception, a weekend duty consisted of at least 24 hours of work.

Many inspections and periodic maintenance were required to maintain the SR-71. A Pre-flight (PR) inspection was performed within 24 hours prior to each flight. This inspection took roughly four hours, and required four mechanics. During the launch, a Crew Chief would perform what we called the "launch supervisory" inspection. This was usually a Technical Sergeant (TSgt) or above, and he generally made sure all went well, and identified any problems. He also accompanied the aircraft to the End Of Runway (EOR) to perform the last chance inspection. During all aircraft recovery operations, a Crew Chief would perform the "recovery supervisory" inspection. During this, he generally checks the condition of the aircraft, and searches for leaks and problem areas. After each flight, a Basic Post-flight (BPO) inspection is performed. Itís a bit more in depth than the PR, but involves the same amount of people and time. Due to the high temperatures the aircraft encounters, a Hourly Post-flight (HPO) inspection is required every 25 flying hours. At BAFB, this was usually after every sixth or seventh flight. This inspection consists of some in depth examinations and a complete aircraft lubrication. Additional inspections are performed every 50 hours. Each 25 hour HPO took about 24 to 36 hours, and each 50 hour HPO took about 48 to 72 hours. This inspection process continued until the aircraft reached the 400 hour HPO. This was a full blown Phased inspection where the entire aircraft and systems were thoroughly inspected. It usually took anywhere from six to nine weeks to accomplish this inspection. Every 800 hours, the aircraft would be sent south to Palmdale California for the Periodic Depot-level Maintenance (PDM) inspection performed by the Lockheed Skunk Works. This six month long inspection is where the aircraft is basically taken apart, inspected, modified, upgraded, put back together, and flight tested.

In the AF, there are several aircraft in the inventory that have become known as "pigs". That's a bad way to say they're maintenance intensive, or in other words, break a lot and are hard to maintain. In the fighter world, it was the F-4 and the F-111. Most of us SR-71 Crew Chiefs came from the fighter world, and completely understand that concept. The SR-71 makes the Phantom and Aardvark look like dream aircraft to work on. I have never worked so hard in my life to keep just one flying. It may sound like a contradiction, but while loving the aircraft, we also cursed it a lot! But let just one person try to call my SR a "pig", and a fight would probably result. While other Crew Chiefs hope for Code-1 (no problems) sorties, we were elated when it landed Code-2 (minor problems). But usually, Code-3 (broke) was the norm. Even on those rare occasions where she did fly good, many times the Digital Mission Recording System (DMRS) tapes would show problems that needed to be corrected, especially on Friday nights it seemed. There were some consistent problems that really stand out. First and foremost, DAFCS. Those inlets were hard to maintain, and as good as those technicians were, we continually had problems. I can't really single out one specific area of that system of concern, but it's something we learned to live with, and spent many hours getting to know the DAFCS folks. Another big problem was heat related engine and electrical problems. I say "heat related" because these problems usually only occurred at speeds above mach three, and then went away once it slowed down below 2.8. How do you troubleshoot those problems? There's no was to duplicate that condition on the ground. We would end up simply changing the most likely component (shot-gunning) that could cause the problem. Really, there was nothing much more we could do than that. Due to the stress of mach three flight, we would always find many structural defects after each flight. I'm generally talking about popped rivets, cracked panels, and delaminations. It wasn't uncommon to have twenty or thirty discrepancies found on each BPO inspection. Another problem occurred when trying to pin-point leaks. With oil, fuel, and hydraulic fluids being the same color, it was difficult to actually see what was leaking. Lets face it, as leaky as she is, there's already a little bit of fuel everywhere.

Introduction     Acronyms & Abbreviations     SR-71 Maintenance     A Typical SR-71 Maintenance Process

SR-71 Deactivation     The U-2 World     War Stories     SR-71 vs U-2     Conclusion     My Biography

© Christopher W. Bennett

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