Operation and troubleshooting
By Greg Napert
The Rolls-Royce RB211-535 is a very popular engine used on around 70 percent of the Boeing 757 aircraft in commercial service.
The engine was introduced in the late Ô70s as part of a line of RB211 engines in service since the early Ô60s. The RB211 family of engines is used on a wide range of aircraft including the A330, Boeing 747, 757, 767 and 777, Lockheed TriStar, and the Tupolev TU204 airliner.
All of the engines in the RB211 family have two unique distinguishing design features: they are divided into seven or eight distinct modules that can be maintained and overhauled separately, and they use a three shaft arrangement. The triple spool layout consists of a single stage low pressure (LP) fan driven by its own turbine. This spool is in turn driven by a twin spool gas generator comprising the intermediate pressure (IP) and high pressure (HP) spools.
The core compressor of a twin spool engine is also relatively long comprising many stages. The speed requirements of the large blades at the front of the compressor can often be significantly different to those of the small blades at the rear of the compressor when the engine runs at power settings other than the one chosen to optimize the compressor performance.
John Masella, senior instructor for Rolls-Royce Product Support Canada says that this three shaft design offers significant advantages over the two-shaft design offered by competitors.
According to Masella, "Many people in the industry point to this three spool design and say it must be overly complex. They point to all of the bearings on all of the shafts as being a problem. In fact, the three shafts offer performance advantages that far outweigh the complexity of the mechanical arrangement.The wide chord fans offer repairablity due to the elimination of the center support shroud or "clappers." Individual fan blades can be changed without removal of adjacent blades.
"A twin spool engine contains a spool that is running at low speed which consists of a fan on one end, with a booster behind it that is connected to the same shaft, and power turbines at the other end which drive the fan/booster. The booster behind the fan is rather inefficient, since it is tied to the speed limitations of the large diameter fan. The core compressor on a twin spool, further, is a very long series of blades that starts at a relatively long diameter and ends up very short. Again, these are all tied together and are limited by the long compressor blade at one end, and the short blades at the other."
The three shaft layout, on the other hand, is designed so that the low pressure fan (LP), intermediate pressure (IP) and high pressure (HP) spools all turn individually. "The advantage here is that since they rotate independently, each compressor operates at it's optimum speed over a greater range of engine thrusts. You might have the fan operating at 4,000 rpm, the intermediate at 7,000 rpm and the high pressure spool at 10,000 rpm. Since the compressor operates more efficiently, fewer stages are required to achieve the required pressures," says Masella.
Additionally, the requirements for variable stator vanes is reduced, which further reduces the complexity of the engine.
As a result, the fewer required compressor and turbine stages yields a shorter engine. "A shorter engine means the shafts are shorter, the casings are shorter, everything is shorter and stiffer, which means the engine tends to flex less along its longitudinal axis and this in turn allows the compressor tip clearances to be maintained. This has proven to be significant because if you measure the fuel burn deterioration over time on the RB211 as compared with similar competitive engines, this engine deteriorates at half the rate of the competitors' engines," explains Masella.
Another advantage of the compact three-spool arrangement is the intermediate compressor stator casing. The casing is mounted in a titanium drum, and this drum is responsible for absorbing all the mechanical loads and forces, which relieves the stator casing from any loads other than air pressures. As a result, the stator casing does not flex, which again means that the compressor tip clearances are maintained.
Masella says, "Of course, being shorter can also mean being significantly lighter — in the case of the Trent, thousands of pounds less than competitive engines in its thrust class."The HP system (Module 41) is the heart of the engine producing the highest amount of heat. It's the "hot section" of the engine and typically requires the most rework.
Beginning with the RB211-535E4, Rolls-Royce introduced yet another significant technological advantage — the wide-chord fan. The wide-chord fan technology offers a clean airfoil surface with no center support shroud. Its large surface area also means the blade is shorter and stiffer which makes it more resistant to impact damage from FOD. Additionally, the lack of a center support shroud gives the technician in the field the ability to remove the fan blades individually — this cannot be done with other fans. If a blade is damaged, you simply install a new blade, perform a calculation to determine a change in trim weights, do a ground run, check the vibration, and you're ready to go.
"It's really hard to argue with the technology when you look at the performance advantages and weight savings of the latest in the 211 family, the Trent. This RB211 design offers significant performance characteristics along with record-breaking time on wing. An RB211-535E4 engine is currently running on a 757 which has over 34,000 hours on it.
All RB211s employ a pressure ratio control system, explains Masella.
"When the pilot moves the throttle lever, he or she is making a mechanical input into a hydromechanical governor on the engine gearbox. But instead of asking for a speed datum on one of the shafts, as with hydromechanical units, the pilot is asking for the governor in the fuel flow regulator to set a pressure ratio in the compressor.
"So every throttle position setting on these aircraft is related to a pressure ratio inside the engine. In actual terms, they are asking the fuel system to control the pressure ratio of the compressor. In fact, what the pilot is really setting is the thrust, or engine rating at climb, cruise, or takeoff. The advantage of this type of system is that as the aircraft climbs, the engine continues to maintain the thrust by changing the pressure ratio — compensating for altitude changes as the aircraft climbs. The fuel system automatically reschedules the pressure ratio which results in a change in fuel flow, pressure ratio, thrust and shaft speed. The engine rating, however, remains constant. Once the rating is selected (Climb, Takeoff, Cruise, etc.), the fuel system automatically maintains that rating based on a predetermined schedule, calculated using atmospheric conditions and throttle lever positioning," says Masella.
The schedule is maintained through the calibration of the metering unit. So the metering system needs to monitor the altitude and it does this through the use of a pressure probe in the engine.
Masella says, "Because of the efficiency of the compressors and the reduced number of variable vanes, the only thing that is needed to assist with compressor changes is a fairly simple variable bleed valve system."
The 211 contains a set of six very reliable bleed valves which are controlled by five solenoids that receive their instructions from a processor called a bleed valve control unit. This unit constantly calculates the air mass flow and pressure ratio through the compressor and schedules the bleed valves accordingly. The main inputs are the N2 rpm, the IP compressor speed, T2 temperature and throttle lever position.
Maintenance on the bleed valve is fairly simple due to the fact that it is microprocessor controlled.
"We have had some problems in the past but they were related to programming problems and were easily resolved through program updates. Additionally, there was data that was interpreted in the field as problematic but was really not. We have upgraded the interface between the technician and the bleed valve control unit with a more user-friendly interface."
Operation and troubleshooting
By Greg Napert
The basic schedule for on-wing RB211 maintenance provided by Rolls-Royce is referred to as the extended maintenance plan (EMP). Each schedule is tailored for each individual customer depending on the type of aircraft they are operating and the type of operation (i.e. commercial transport or cargo).
The EMP also covers the rework policy in the shop. There are some unique advantages of the modular design, but some opportunities to abuse the modular concept as well.
"The beauty of the modular engine is when something breaks you can replace the module," says Masella. "But you've got to consider the other modules before you replace or overhaul any single module. If you overhaul one of the modules, and do nothing to the others which may be close to removal as well, the module that you overhauled may be removed from service as soon as one of the other modules requires overhaul. You may end up pulling an engine several times within a few months because of various overhaul requirements in the different modules. This is a very inefficient way to do things.
"For this reason, you need an engine management program. You need to determine the condition of the remaining modules before returning the engine to service."
"The engine management program addresses a particular philosophy of controlling the amount of rework on even serviceable modules when the engine comes into the shop. This is where the term 'Soft Life' comes from. The modules may or may not have a failed part, but regardless, the extent of rework/repair is based on the unique soft life of each module. It may be advantageous to overhaul a module, for example, even though it has some service life remaining if it will mean the engine can stay on the aircraft for a few thousand hours more."
"For the operator that has a pool of spare engines and modules, we try to mix and match to produce an engine with the longest available service life," says Masella.
The on-wing tasks covered by the EMP address such items as a walk around, checking the EICAS (Engine Indicating and Crew Alerting System) which logs maintenance messages, oil system checks, a general inspection around the engine.
Masella says, "There are really no unusual items. Obviously, at some point during the engine's life, you have to perform a hot-end borescope, change fuel filters, etc. The EICAS can also be very helpful as it is programmed to review any faults and categorize them into either status messages, which are displayed in the cockpit, or maintenance messages. The maintenance display of the EICAS on the RB211 right now displays only maintenance messages — there are currently no status messages built in. The main difference between status and maintenance messages is that status messages are a no-go item. Maintenance messages can be carried until they can be addressed by maintenance.
"One tip I like to recommend for monitoring the engine with EICAS is to ask the flight crew to log any large EGT splits over 30 degrees. Typically, any split should correspond to error messages in the EICAS which will then give you clues as to items that might be failing," he says.
On any complex mechanical system, there will inevitably be in-service problems that develop. As problem-free as the RB211 is, it is no exception to the rule.
Three recurring problems the technician might be exposed to, related to the RB211, are fan vibration, high oil consumption, and throttle stagger.
According to Masella, "There are fault isolation procedures in the manuals for these problems, and the procedures for these symptoms have recently been thoroughly revised. Some additional tests have been put in the manual, and the best information available is out there now in terms of troubleshooting.
"Rolls-Royce's RB211-535E4 fan trim balancing procedures have also undergone a number of evolutions with quite successful results. The fan is dynamically balanced in the shop as a module, and then procedures have been developed to further trim balance the fan on-wing. One of the reasons for in-field balance problems can be erosion of the anti-fret coating at the dovetail root. Another item that can cause a problem is the rubber seal in the annulus fillers between the blades. These seals can become lodged out of position and cause balance problems as well. We have a procedure that we refer to as the 'Credit Card Check,' in which we slide a card size piece of plastic (preferably a gold or platinum card) between the blade and the annulus filler to make sure the seal is pushing downward. Don't ever poke in these spaces with metal or screwdrivers or anything that would damage the rubber seal," he explains.
Masella says, "The hollow titanium fan blades allow very little in the way of repairs or dressing, however. The dressing that is allowed requires the use of special silicon-carbide tools. Typically, any significant nicks, dings, or any other type of repair will require a blade change."
"There is also an ongoing modification program to improve the structure of the airfoil. We did have one blade fail in a 747 and this resulted in the requirement for an ultrasonic test on the airfoil. The test uses an ultrasonic transducer that sends a pulse down the airfoil. The cavity inside the airfoil is evacuated of air when it is manufactured, so if a crack penetrates the blade, it allows air into the cavity, and the dynamic response of the ultrasound probe will be different and the technician will be alerted to the crack. The ultrasonic procedure needs to be accomplished at overhaul and on the wing at recommended intervals. These intervals continue to be reduced as we become more and more comfortable with the blade," he says.
Oil related problems on the RB211-535E4 have multiple origins.
First, high oil consumption can come from extended low power operation. "All the bearing compartments on the RB211 are sealed with air, so operating the engine at low power simply means that the available sealing air is less than it would be at high power or cruise settings. It has been shown that normal oil consumption can double when the engine is run at low power for an extended period of time, such as when taxiing or just sitting at the gate. For the airlines, it is not unusual to taxi or sit for hours at low power. If the same engine is now also burdened with slightly higher consumption than normal due to sealing problems, the combination can result in quite significant losses on the ground. For operators experiencing this problem, we simply suggest they bump up the throttle a bit and maintain a slightly higher EPR during taxi.
"Apart from oil consumption, there is also occasional oil wetting of the turbine. This is an area being addressed by engineering action. The symptom is puddling of oil at the base of the turbine or oil running down turbine blades, which can be a bit disconcerting. There are fundamental design changes being incorporated into the bearing compartment — such as improved scavenging, an introduction of a new seal, etc."
"Oil wetting can also be the cause of smoke on shutdown," Masella explains. "This can be a bit disconcerting as well, but it is simply small quantities of oil running down on the turbine blades. Certainly, you want to do something to fix this, appearance aside, the smoke is not a cause of rejection itself. Unfortunately, there is really nothing that can be done about this on the wing."Rolls has the ability to perform repairs, such as replacement of the Teflon® jacket surrounding the fan containment ring, as well as overhaul of all RB211 series engines.
"The last area where we've experienced some problems with oil leaks on the 535E4 is in the shroud surrounding the gearbox radial drive shaft. The two halves of the shroud are joined with a seal that can become dislodged due to vibration. This is a serious problem as a leak here can result in an inflight shutdown. Right now, the recommendation is to inspect this seal every 100 hours to be sure it is in place."
"Another interesting reported problem for the technician to solve is throttle stagger," says Masella. "The beauty of electronic fuel control is that it virtually eliminates throttle stagger. However, if you introduce a sufficiently large fuel scheduling error in the Woodward governor even with the electronic control, there is going to be some throttle stagger. Typically, of course when you have throttle stagger, you need to look at the fuel system. The P1 sensing system varies fuel flow so if there is stagger, it's not uncommon to change the fuel flow governor."
"And that's what happened recently on a 535E4 engine. Except, after having fixed the problem with a new fuel governor, the problem returned, and it did that twice again, until the last circumstance when the throttle stagger occurred rather suddenly and the engine surged. In the end, what we discovered is that a gearbox failure had caused a vibration that caused a mechanical failure of the P1 mechanism in the fuel flow governor."
"So the fundamental underlying cause was a gearbox failure — this kept causing the failure of the same part in the fuel flow governor. Of course, there are vibration sensors on the engine, but in this case the frequency was outside the range that is typically monitored. This problem produced a good lesson in troubleshooting — look at underlying causes for failures, don't just fix what's obvious."
Despite these relatively few challenges, you will find the RB211 is one of the most dependable engines in its class. Adherence to the EMP and good service techniques will mean years of trouble-free operation for the RB211.