Dangerous Rudder

Dangerous Rudder

Dangerous Rudder

Traditional oral exams begin with a cavalcade of numbers. The multitude of airspeeds, weights and operating restrictions that are regurgitated inevitably succumb to mist within a week (humans are horrible at remembering arbitrary numbers). Fortunately, most of the important stuff is depicted by color-coding on gauges. Everyone knows that red represents a restriction, yellow indicates caution, and green means go. Glass cockpits with integrated electronics have further reduced the burden of memorization – computers can deal with the details while humans manage the big picture.   

The first airline that I worked for (now defunct) demanded an unholy amount of memorization. We had to recount every limitation on the aircraft, as well as 80 steps split between two dozen different procedures (we had five different go-around profiles and befuddlingly had to memorize how to enter a stall). Faithfully recalling two pages worth of memory items was improbable during live-fire training and impossible during an actual emergency. We were at the tail-end of the industry transition away from memorization. Quick reference checklists are much more reliable. 

An engine fire (as an extreme example) does not require an immediate response. Flames in flight are scary, but firewalls protect the occupants for a period of time – at least long enough to ensure aircraft control prior to running the appropriate procedure. Obviously, a fire is a condition that must be addressed, but it is all too easy to “do the wrong thing rapidly” in a rushed response to a high priority item. More than a few crashes have resulted from scurrying pilots shutting down the good engine.

Pilots should restrict memory items to issues that immediately affect the stability of flight. This does not mean that a pilot should not “fix the obvious” (if you switch fuel tanks and the engine begins to run rough, for example), but that deliberation is often the better solution to in-flight disturbances. 

Flight Controls

Numerous studies indicate that stick-and-rudder skills are also enhanced when they are a part of a deliberative process. All too often pilots react impulsively in response to relatively benign disturbances. There are many accidents resulting from confused pilots reacting without thinking, inputting control forces that ultimately doomed the aircraft.  

The failure to fly deliberately has produced a history of inappropriate inputs in all three axes of flight, but the worst offender has been the rudder. It is one of the most confounding devices on an aircraft. The controls are buried beneath the pilot’s feet, so it can be difficult for an observer to determine how a pilot is manipulating the pedals. This reduces the ability for instructors (or more experienced copilots) to pass along corrections for ineffective rudder technique. Sometimes pilots are released into the wild sans a substantial idea of how to use one of the biggest control surfaces on an aircraft. 

Over time we develop some ideas about the rudder. Pilots of taildraggers cannot stop yapping about it (it is hard to forget the experience of a ground-loop). It is used in props to counteract p-factor and as an aerodynamic cudgel to eliminate excess energy on final. In aircraft from LSAs to A380s, it is the means to maintain directional control in crosswind conditions. The rudder is alternately the most demanding and the most dangerous device aboard an aircraft. 

Va is the maximum speed at which full deflection of the flight controls can be commanded without undue risk of structural failure. It is an arbitrary number that should be memorized. As important as it is, Va comes with a rather important caveat: Flight controls are only protected when inputs are initiated from the neutral position. Alternating deflections (stomping on the right rudder and then stomping on the left) can result in structural failure – even at speeds well below Va. Although this caveat technically applies to the elevator and ailerons as well, it is the rudder that is most at risk in the event of oscillatory inputs.  

Enhancing the danger is the fact that the rudder is one of the least predictable devices on an aircraft. Both the ailerons and elevator are fairly linear – a given control force will result in a fairly predictable rate of roll or pitch. The rudder is quite a bit more finicky. If the aircraft is in a slip, rudder towards the side of the slip will have to overcome the aerodynamic forces already acting against the vertical stabilizer, and a little bit of rudder will have little effect. If rudder is opposite the direction of the slip, the aerodynamic force of the rudder and vertical stabilizer will sum into a large rate of yaw. Thus the same amount of pressure on a rudder pedal can
produce significantly different results (a notable percentage of pilots reported experiencing “unexpected rudder characteristics” in an FAA survey). 

This ambiguity greatly increases the probability that a pilot will fall into the trap of divergent oscillation, cycling from left to right rudder at an increasingly dangerous rate. The yaw generated from an engine failure on a multi-engine aircraft, for example, can create a startle response inappropriate for the situation. Kicking the rudder in alternating directions can place massive forces on the vertical stabilizer risking structural failure. The scale of those forces is insidious due to the fact that pilots often have no kinesthetic notion that they are occurring (a rudder may experience thousands of pounds of force, while the rotational acceleration felt by the pilot is relatively minor).

The failure to fly deliberately has produced a history of inappropriate inputs in all three axes of flight, but the worst offender has been the rudder.
It is one of the most confounding
devices on an aircraft.

FAA Technical Report

The issue of rudder usage was central to an FAA report released in October of 2010. The survey focused on transport category aircraft but is applicable to high-performance passenger aircraft as well. The survey focused on unusual attitudes, with 914 pilots relating various experiences encountered over the course of their career. 

Nearly two-thirds of pilots reported unusual attitudes during low-level flight associated with takeoff, landing and initial climb. The vast majority were attributed to wake vortex encounters (62 percent) and atmospheric disturbances (21 percent). It is important to note that nearly all respondents flew aircraft that weighed in excess of 100,000 pounds. Wake turbulence is not a problem for only small aircraft. 

With wake turbulence making up the majority of disclosures, it was unsurprising that roll upsets were predominantly described. Interestingly, 56 percent of pilots reported using the rudder in response to these roll events. This almost exactly matches the number of pilots who indicated that they had received aerobatic training at some point in their life. Aerobatic flight demands a substantial amount of rudder coordination, sometimes applied quite aggressively. The law of primacy in training is apparent here as pilots routinely transfer techniques learned in earlier flying experiences to aircraft for which they may be inappropriate and sometimes dangerous. 

Propellers have not changed much over time – feathering can be a matter of life or death.

In many cases, the respondents described using rudder in a manner inconsistent with industry and manufacturer guidance. The final report also noted that pilots tend to consider an aircraft “upset” based on motion (g-forces) as opposed to pitch or bank angles. Pilots perceived the immediate need to intervene at pitch and bank angles substantially less than the “unusual attitude” definition of 25-degrees nose up, 10-degrees nose down, or 45-degrees of bank.   

While preventative awareness of pitch and bank is commendable, aggressive reactions are generally unnecessary. A benign atmospheric force (or wake turbulence) often disturbs a flight only momentarily. Passenger aircraft are almost uniformly designed to be stable so that a disturbance will automatically result in a restorative force. Sudden g-forces can create the dreaded startle response, which routinely produces inappropriate reactions to otherwise mild upsets. One-sixth of pilots admitted to inappropriately over-controlling or making inputs in the wrong direction in response to sudden stimuli. Pitch excursions based on improper pilot input were rare (likely due to the kinesthetic of g-force feedback). Inappropriate roll commands were more likely. The rudder was the greatest offender.

Troublingly, many of the erroneous rudder inputs were calculated to have exceeded certification criteria. While none of them exceeded the ultimate structural load limits, they nonetheless transgressed the margin of safety built into the certification process. The reality of this was reinforced by a companion simulator study that revealed the tendency for pilots to over-control the rudder (exposing the vertical stabilizer to large g-forces) with the ham-fisted application of rudder inputs. Each type of aircraft is unique, and it is incumbent on pilots (and training programs) to develop type specific techniques appropriate for the particular airframe. In general, modern swept-wing aircraft (those with active yaw dampers) do not require much rudder input outside of asymmetric thrust or crosswind conditions. 

“Between 1994 and 2003 there were [multiple aircraft] accidents across the globe which resulted in more than 2,100 fatalities – all the result of aircraft upsets.” – FAA “Perspectives of Directional Control Events”

From the NTSB Database

Overusing the rudder can have devastating consequences, yet it is also dangerous to ignore it. The rudder conveys important information to a pilot, and proficiency in interpreting that information can be indispensable to successfully resolving an in-flight disturbance. An overemphasis on rudder use is uncalled for, but the failure to develop proficiency on it can be reckless. 

The crash of a Cessna 414A in picturesque Kahului, Hawaii, had (as most accidents do) many factors. The Part 91 aircraft was carrying three occupants, all of whom died. The left engine failed for undetermined reasons while the aircraft was preparing for landing. The recovered aircraft was configured with the flaps fully extended and the gear down. Both propellers were found in a low pitch (not feathered) setting. The crash occurred 0.6 miles west of the approach end of the runway.

Very few multi-engine aircraft can maintain altitude with an engine inoperative, gear down and flaps fully extended. Feathering the propeller on the inoperative engine is also paramount to aircraft performance on a propeller-driven aircraft. In the mental fog that accompanied the mechanical malfunction, the pilot failed to account for the drag that the aircraft was encountering as a result of its configuration. Controls in the aircraft allowed a quick remedy for gear, flaps and propeller settings – and would have substantially increased performance, allowing the remaining 310-horsepower engine to complete the emergency landing. 

As airspeed decayed below Vmc, a pronounced roll into the failed engine would have inevitably developed. The ailerons do not have the control authority to manage this at slow airspeeds, requiring an increasing amount of rudder to counter the roll. The rudder shouts at the pilot to lower the nose, reconfigure and climb away – or lower the nose and crash right-side-up if no better option is available. Yet all too often, the pilot, oblivious to the information being conveyed by the rudder, keeps increasing the elevator until the roll becomes a Vmc spin. Too many pilots have succumbed to Vmc rolls throughout the history of multi-engine flight. The results are nearly always fatal. 

The rudder can be confounding and is likely the most overlooked device on an airframe. Its importance is integral to pilot proficiency, but it is also in danger of being overstated. In one of those little contradictions common to life, the proper approach is to know your rudder well – just try not to use it too much.       

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2 Comments

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    Karen and Stan Dunn January 30, 2021 at 7:43 pm

    Wonderful article. You are saving lives, Stan! We are impressed and love your writing.

  • Avatar
    Mark A. Murphy February 6, 2021 at 12:43 am

    Well Dunn, Stan. (couldn’t resist)

    meooowwwww.

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