Hello B737-pilots,

During a Boeing Flight Ops symposium, a lot was said about the rudder PCU of the B737. One of the items discussed was pilot knowledge of the present system (with RPR and digital yaw damper).

I compiled questions for the B737 pilots in our company (with answers), that I am happy to share with you. The answers are at the end, because not all of this is in the Boeing manuals.

Q1. Are the rudder pedals reflecting the correct rudder position if the yaw damper is off?

A1. The rudder pedals give an accurate picture of the rudder position (even if the rudder failed). The yaw damper input is not fed into the rudder pedals, but the yaw damper authority is limited to 3 degrees.

Q2. Is the travel of the rudder pedals on the ground (about 4 inches during the flight control check) the same as in flight at 300 KIAS (with and without RPR)?

A2. The rudder has a blow-down feature. While you can move the pedals 4 inches on the ground, at high speed the pedals will move only 2 inches reflecting the limited rudder travel. Even without RPR, rudder travel at high speed is limited.

Q3. Is there an indication in the cockpit during normal flight if the RPR fails to reduce the pressure to the rudder?

A3. No. If the RPR fails to reduce the pressure after take off, there is no indication in the cockpit. The aircraft may continue to operate until the next major inspection (5000hr, can be 2 years later) using the lower block speeds. According to Boeing, the chance of an additional dual failure resulting in a rudder hard-over in that timeframe is statistically not significant. Obviously, Boeing is being put under pressure to change this philosophy.

Q4. If during approach, at around 700ft RA, the "System A flight control low pressure light" illuminates, should you just continue and land or go around and investigate?

A4. Land the aircraft. Write up the malfunction after landing. If the RPR fails to increase the hydraulic A pressure during approach at 700ft RA, the "System A flight control low pressure light" illuminates. The danger is the combination of this failure + engine failure: you may not have sufficient rudder to recover. You land to decrease the chance of suffering an engine failure in flight, as you can do nothing to restore full hydraulic pressure.

Q5. If the yaw damper fails in flight, does this affect the RPR?

A5. No. If the yaw damper fails, the RPR is not affected.

Q6. Using the MEL, why do you increase block speeds if maintenance labels the yaw damper INOP?

A6. If the yaw damper is INOP, maintenance has to deactivate the RPR because it is in the same black box. So the MEL requires you to increase block speeds.

Q7. With a rudder hard-over at very low speed, what is the single most important factor that a pilot can change to decrease the cross-over speed and regain control?

A7. Decrease angle of attack. Cross-over speed increases dramatically at higher angle of attack. If you ever experience a rudder hard-over at low speed, (turn with the ailerons and) push the stick forward to decrease AOA.

Q8. Is the B737, with new PCU and RPR installed, protected from a rudder hard-over during take off until flap retraction?

A8. No. The RPR still provides full pressure and you are well below cross-over speed. Boeing is under pressure to review this.

Q9. During taxi, you feel "kicks" in the rudder pedals, what do you do?

A9. Return to the gate immediately, rudder kicks are a symptom of an impending PCU failure.

Q10. What is, according to you, the most efficient way to check the rudder during taxi to discover an impending PCU failure: a slow gradual movement to full travel left and right or the same movement but rather fast?

A10. A rather fast movement full left and right. Some PCU failure modes can not be detected if the control check is done very smoothly (info coming from USAir).

Note: if you have a non-normal rudder event, do not shut down the electric hydraulic pumps before maintenance can investigate, but keep the entire system pressurized. Some failures can disappear once the pressure is relieved from the system only to re-occur at a much later stage.


For the record: Boeing is designing a new rudder system (the third) to be installed between 2003 and 2008 in all B737's (3840 aircraft, increasing with 24 a month).


Capt. Andre Berger
Training manager
Sobelair
andre.berger@advalvas.be