Part 2:
A line check airman with a major airline noted that some pilots, when practicing roll upset recovery in the simulator, will "fight initially with aileron, but at the panic point they tend to stomp rudder, while some are better with coordinated controls. Many have their feet flat on the floor on climbout--it's very disturbing to see this."
Strong rudder use is seen in service as well as in the simulator. In 1997, an American Airlines crew used large rudder motions in recovering from a stall in an A300-600 ( AW&ST Mar. 18, p. 47). Recent calculations showed that ultimate load was exceeded on the fin.
Manufacturers are concerned that large rudder use in upset recovery can result in overcontrol. "If you're using rudder to get out of a roll, by the time the wings go level you've built up such a large yaw that you go shooting out the other side," one official said.
Investigators reportedly think the rudder motions started in Flight 587 after it encountered the wake of the preceding 747. At their speed, just the yaw acceleration from full rudder would create roughly 0.5g lateral force in the cockpit, a level that might be higher than expected and cause some confusion.
It must be noted that the NTSB has not determined that the pilot moved the rudder, and is actively considering a system malfunction. In one case investigated by the NTSB, an autopilot flaw caused large rudder motions and the board has not ruled out this possibility (see p. 47).
Rudder controls have evolved from manual actuation. Aerodynamicists view rudder properties nondimensionally--a rudder deflection of, say, 10 deg. will have a certain hinge moment coefficient that resists deflection, and will create a lift coefficient on the tail. These coefficients are multiplied by the dynamic pressure, or the square of the equivalent airspeed, to calculate the actual moments and forces.
MOST MANUAL systems have constant gearing between the pedal and rudder, and the pedal becomes stiffer as the square of the airspeed. The pedal force to hold 10 deg. of rudder may be 20 lb. at 125 kt. but grow to 80 lb. at 250 kt.
The resulting fin yaw force also grows as the square of the airspeed. To the first order, the pedal force is a measure of the fin loads over a wide range of airspeeds--what you feel is what you get. "Forces are a clue to limit loads," the research pilot said.
Another nice feature of manual systems is that pedal sensitivity remains constant with varying airspeed. This is because yaw acceleration and pedal force grow equally with airspeed, though rate damping becomes an important effect at higher speeds.
Higher weights and speeds forced hydraulic systems upon manufacturers. Designers realized that an irreversible hydraulic actuator that did not feed hinge moments back to the pilot would result in overcontrol and structural damage. No longer does "what you feel is what you get" hold true. They devised artificial feel systems to give a semblance of manual feel, including in the yaw axis.
Boeing's 1947-vintage B-47 bomber had a "Q bellows" that mechanically sensed dynamic pressure and made the pedals stiffer with speed, like a manual rudder. Most 707 versions have feel provided by an aerodynamic tab on the rudder and a Q bellows, and have a similar manual feel.
But while newer aircraft continued to have pitch sensitivity that was roughly equivalent to manual, rudder control systems took a different evolution that strayed from the manual roots. Clever design meant the pedals were no longer needed to coordinate every turn, and they became an almost vestigial control for occasional use at low speed.
Newer systems fall into two broad groups with a few common features. One similarity is that pedal deflection works against a fixed spring. Unlike manual feel, it does not become stiffer with speed. Another common feature is that both groups limit rudder travel at high speed to prevent overloading the fin. However, the limiters may not protect against rapid rudder motions in a sideslip ( AW&ST Jan. 21, p. 24).
One group has fixed pedal-to-rudder gearing with rudder deflection limited at high speed either by blowdown (actuators overcome by airloads) or by variable mechanical stops. As the airplane goes faster there is less pedal travel available. This is the "fixed ratio" group.
The other group has variable pedal-to-rudder gearing so that full pedal travel is always available but results in less and less rudder motion. This is the "ratio changer" group.
THE FIXED RATIO systems become increasingly sensitive because a given rudder position causes more acceleration at high speed with no rise in pedal deflection or force. "Having sensitivity get larger with speed seems opposite to what you want," the research pilot said. "Why on Earth would you do that?" Also, the limiter chops pedal travel to shorter amounts, making the breakout force an increasingly dominant player just when the pedal is becoming more sensitive.
A Boeing official noted that roll also becomes more sensitive with airspeed, but pilots have more practice in this axis than in yaw, and aeroelastic and rate damping effects mitigate aileron sensitivity more strongly than rudder.
The ratio changer group typically shifts the pedal-to-rudder gearing in proportion with dynamic pressure, making the sensitivity approximately constant with speed, like manual feel. And because full pedal throw is always needed to reach the rudder limit, the breakout force remains relatively small.
Manufacturers have had different design evolutions. Douglas has used fixed-ratio systems on all its jets, limiting motion with variable stops or blowdown. Boeing used fixed ratio with blowdown limiting on the 727 and 737, then switched to the ratio changer for the 747 and subsequent aircraft, finally enshrining it in software on the 777 fly-by-wire rudder. Airbus started with the ratio changer (called Variable Lever Arm) on the A300B2/B4, then switched to a fixed-ratio variable-stop system (called Rudder Travel Limiter) on the A310 and subsequent aircraft, including the A300-600. Similarly, the variable-stop system is now implemented in software on Airbus' first fly-by-wire rudder on the A340-600.
Airbus says the Rudder Travel Limiter (RTL) has been thoroughly flight tested "and does not lead to overcontrol." "At high speeds, the rudder deflections required to counter major lateral asymmetries are small and so are the rudder pedal inputs," the company said, noting that this is similar to pitch and roll controls where deflections also reduce with speed. "From a technical standpoint, the benefit of the RTL is that it is simpler than the Variable Lever Arm, and hence overall more reliable."
RUDDER LIMITERS AND PEDAL SENSITIVITY
Airplane Limiter Type Pedal sensitivity trend with airspeed*
BOEING
707 Blowdown (w/deboost) Approximately constant sensitivity from increasingly-stiff feel provided by tab and Q bellows. Similar to manual feel. Fixed ratio.
727 Blowdown (w/deboost) More sensitive at high speed. Fixed ratio.
737 Blowdown (w/deboost) More sensitive at high speed. Fixed ratio. All 737s used to have constant hydraulic pressure but deboost was part of actuator modifications in late 1990s.
747 Ratio changer Approx. constant sensitivity.
757 Ratio changer (blowdown backup) Approx. constant sensitivity. If ratio changer fails one actuator is turned off and rudder becomes blowdown-limited.
767 Ratio changer (blowdown backup) Approx. constant sensitivity. Same as 757.
777 Ratio changer (blowdown backup) Approx. constant sensitivity. First Boeing fly-by-wire rudder implements ratio changer in software. Deboost occurs if in a backup software mode.
DOUGLAS
DC-8 Blowdown and variable stop More sensitive at high speed. Fixed ratio. Deboost used at cruise speed.
DC-9 &MD-80/90 Variable stop More sensitive at high speed. Fixed ratio.
DC-10 &MD-11 Blowdown More sensitive at high speed. Fixed ratio. No deboost.
AIRBUS
A300B2/B4 Ratio changer Approx. constant sensitivity.
A300-600 Variable stop. More sensitive at high speed. Fixed ratio.
A310 Variable stop. (A340-500/600 with first Airbus fly-by-wire rudder is also variable stop).
A320 Variable stop.
A330/340 Variable stop.
NOTES:
*Pedal sensitivity is yaw acceleration per incremental pedal force.
--Blowdown-limited means the hydraulic actuators don't have enough force to fully deflect the rudder at higher speeds. Often this is assured by deboosting (reducing) the hydraulic pressure with airspeed or flap setting.