P3 air gasket leaks
Another problem that can challenge technicians and create troublesome operational characteristics on the Tay is related to P3 air gasket leaks. Hewitt explains, "The fuel flow regulating system on the engine is such that you set the power lever at an HP speed setting and the engine will automatically maintain that HP setting with minimal throttle lever movement. The fuel flow regulator controls the HP setting and senses various delivery air pressures throughout the engine such as P2.6 and P3 — that's HP1 inlet pressure and HP12 delivery pressure. If you get a leak at either of those air signals to the fuel flow regulator, it will give you a false scheduling for your particular climb performance, cruise performance, or takeoff.
"Any of these could cause the fuel flow regulator to malschedule. Some external seals on the fuel flow regulator have generated leaks, which usually show up as slow acceleration or "hunting throttles" with autothrottle selected. Hunting throttles occur when throttles are set at cruise and one throttle hunts back and forth and all of the parameters follow suit. What's really happening at the fuel control is that there may be an uncontrolled air scheduling leak and the unit is trying to correct for that leakage.
"These seals or gaskets, once made from asbestos, held up well in the heat being generated by air coming off the HP compressor and into the fuel flow regulator. Following the ban on asbestos, other materials have been used, but they haven't worked as well.
"Lucas, which manufactures the fuel control, has been working on modifications which should eliminate the occasional air leak problem, but it still exists as we speak.
Hewitt continues, "When any of these symptoms appear, we change these seals in the field right away. We were not allowed to change the seals previously because of the procedures necessary to seat the seals properly. We now have clearance to change the majority of these external seals with the engine installed on the aircraft. Specific procedures for installing these seals involve removing the fuel flow regulator from the engine, installing the new seals, heating the unit in an oven, and heat soaking it for about five hours with the new seals, which are re-torqued throughout the heat soaking process. An alternative is to remove the unit and send it to Lucas and they will perform the procedure and have it back to you in a short time.
"To determine whether or not you have a problem, use a soapy solution and spray it in the general area of the seals with the engine running. If you see small bubbles, you know you have a problem, because even a small amount of leakage can cause a scheduling problem. Some internal seals in the fuel regulator can cause problems which are more difficult to diagnose. Essentially, if you have performance problems or throttle hunting problems, and you can't find a leak externally, you can pretty much deduce that it's an internal malfunction," he says.
Masella adds, "There were some fundamental changes made to the fuel flow regulator which have reduced the number of seals for both the fuel and air, resulting in less leakage. The problem principally is at the inlet connector, so using soapy water at the inlet connector can tell you if it is leaking. A note of caution: next to the inlet connector, there is a small pinhole moisture vent that will produce bubbles during a leak check. Don't mistake this for a leak — it's perfectly normal, but there shouldn't be any bubbles anywhere else. Masella continues, "To repair the seals, you need to remove the P3 block which is removed with four bolts. Keep in mind that if you do that, the P3 block must be returned to the same fuel flow regulator as they are calibrated together as a unit.
Hewitt says, "Quite often if we are unsure of exactly what the problem is, we will change the fuel flow regulator as a matter of course to verify that the problem is with the regulator."
Hewitt continues, "I like to think of the fuel flow regulator as the carburetor of your engine — it houses all the control functions of the engine.
"A problem with hunting throttles can also be attributable to programming of the autothrottle electronics. There have been some software updates and this can also solve the problem. I have seen cases where correcting a hunting autothrottle can require a combination of fixing P3 air leaks and software upgrades."
Increase in LP at the top of climb
Hewitt says, "All Rolls-Royce engines have what we refer to as a Ôbelt and braces' controlling mechanism. There are four of these control mechanisms or Ôlimiters': the LP governor, HP speed, overspeed high turbine gas temperature (TGT) control mechanism, and P3. If the engine goes into an overspeed condition or the temperature rises to the maximum limit, the engine will be controlled automatically.
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