FCIF 18-0440 B-767 Thermal Trim Compensator (B757/767)

Issue Date: 10/22/18 1602 Source: Christopher Frasse Suspense: 10/31/19 Approved: Matt Gandy

The 767 rudder system is equipped with a Thermal Trim Compensator to account for different thermal expansion rates of the rudder, rudder actuator control rods and the vertical stabilizer. The compensator is a bi-metal rod that inputs a small left rudder displacement (bias) until the aircraft has cold soaked at altitude for 60-90 minutes. This rudder bias is NOT shown on the rudder trim indicator. The rudder bias is shown on the status page, but for 1-3 degrees of displacement, it is not very obvious. The obvious indicator is the yoke being displaced to the right of the centered position.

Recently, there has been an increase in the number of 767 AML entries for airplane’s requiring excessive aileron or rudder trim to fly balanced in climb or cruise. FedEx maintenance and Flight Test generated an inquiry to Boeing asking why several newly delivered aircraft were experiencing ‘suspected’ excessive lateral (aileron) and/or directional (rudder) trim AML discrepancies. Boeing suggested that trim discrepancies could be caused by the design of the thermal trim compensator. Additionally, FedEx maintenance performed flight control rig checks on the suspect 767s and the sign off for the AML entries was that the flight controls were in design limits.

Bottom line: Sometimes in the climb and initial cruise, up to 4 degrees of right rudder trim is required to level the yoke, center the ball, and fly in balanced flight. Most of the time however, 1-2 degrees of right rudder trim is sufficient for balanced flight. If it is a short flight, this trim will be required for the entire flight. If cruise is 60 minutes or longer and the aircraft cold soaks at altitude, this trim requirement will slowly return to zero.

The right rudder trim input in the climb by the flight crew, even though it is showing right trim displacement, is actually re-centering the rudder to offset the trim compensator’s initial left rudder bias, and centers the yoke back to ‘0’ displacement. The point of the thermal trim compensator is to minimize trim requirements in cruise at the expense of climb trim for greater fuel efficiency.

FedEx Flight Standards, Flight Test and Boeing agree that we can eliminate most, if not all of Rudder and Aileron Trim AML write-ups by educating our 767 crews to understand the design of the thermal trim compensator in our jets. Pilots thinking of making AML entries for excessive Rudder and/or Aileron trim must consider the design and limitations of the thermal trim compensator before making a logbook entry.

Questions regarding this FCIF may be directed to B757/767 Flight Standards: Captain Chris Frasse, Manager B757/767 Fleet Standards