Boeing A number of unintended pilot induced roll oscillations during the final landing phase of flight have been reported. These have typically occurred at 100 ft (30 metres) AGL or less, with Flaps 30 selected. These oscillations, although abrupt, do not usually involve large bank angles. Analysis shows that they typically occur in gusty wind conditions. Some roll PIO events have been accompanied by abnormally large attitude changes due to gusts and turbulence.
Pilot Induced Oscillations (PIO) are inadvertent, sustained oscillations of the aircraft resulting from interactions between the aircraft and control inputs by the pilot. PIOs are often associated with tasks where the pilot is attempting to precisely and quickly accomplish a flight manoeuvre (such as the final phase of landing).
They can be precipitated by external events such as wind or turbulence, or by flight control characteristics. These events may cause the pilot to apply larger and faster than necessary inputs to the control wheel in an attempt to maintain attitude or correct for an attitude upset. Initial entry into a PIO may be subtle and the pilot flying may not recognise a PIO situation until it is well developed. In a fully developed PIO, pilot control inputs will be out of phase with the aircraft response. When the aircraft is fully out of phase with the control inputs, control wheel inputs will seem to be opposite to the aircraft roll response.
Pilot techniques that utilise abrupt and pulsing control inputs may contribute to a possible PIO. These techniques are not appropriate for large commercial airplanes that cannot respond quickly and stay synchronised with these inputs due to the large mass of the airplane.
Boeing analysis of the reported events have identified some characteristics with which pilots should become familiar:
Roll [Control] Effectiveness. The Boeing 757 with landing flaps has very effective roll control. Roll control effectiveness is greatest at Flaps 30. In addition, the effectiveness of the controls, expressed as roll rate per unit of wheel deflection, is such that roll response at 12 units (60 degrees) is more than twice that experienced at 6 units (30 degrees). Abnormally large control wheel inputs may generate larger than expected roll rates.
Airflow Separation. During recent testing at Flaps 30, it was noted that airflow separation over the outboard trailing edge flaps behind the outboard wing spoiler panels could occur abruptly at approximately 8 units (40 degrees) of wheel input resulting in an abrupt increase in roll control effectiveness. Roll input past this point will continue to increase roll rate normally.
Boeing Service Bulletin 757-57-0058 adds vortex generators on the leading edge of the trailing edge flaps. With the generators installed, airflow detachment over the flap surface occurs less abruptly and roll control is more predictable. The vortex generators are visible on the leading edge of the forward, outboard flap segment when flaps are fully extended.
Vortex generators are fitted to G-JMAA, G-JMAB, GJMCD and G-JMCE and incorporated on all production aircraft from line number 911 and on. The 757-200 has 11 per side and the 757-300 may have 14 per side. The FAA is in the process of mandating an Airworthiness Directive where all B757 aircraft are to be fitted with vortex generators within a 36 month compliance period. Changes to the MEL CDL will include - if more than one vortex generator is missing on either side, use of Flaps 30 for landing is prohibited. There is no performance penalty. When using Flaps 25, strictly maintain the recommended approach speed to help reduce the chance of a tail strike.
Wheel Centring Detent Force. When the control wheel is at the trimmed or neutral position, and force is applied to the control wheel to make a control input, there is a small initial force input that does not result in flight control (spoiler or aileron) motion - a breakout force. Control wheel inputs beyond this “detent force” result in the programmed aileron and spoiler motion. Excessively large control wheel detent forces may contribute to imprecise control wheel inputs. Aircraft should be rigged to the appropriate maintenance document tolerances.
Aileron Actuator Rate Limit. Hydraulic actuators drive all 757 primary control surfaces. If the rate of roll input by the pilot exceeds the maximum rate capability of the aileron actuators, the following effects will occur:
The maximum rate capability of the aileron actuators translates to approximately 125 to 130 degrees/sec at the control wheel.
A solution being considered is to install a new, rate dependent, control wheel force damper below the first officer's control wheel. The control wheel damper will provide a resistive force to both control wheels when the wheel rotation rate reaches approximately 130 degrees/sec. This rate corresponds to the aileron actuator rate limit.
Decreased Lateral Control Wheel Forces. Lateral control forces typically increase with control wheel deflection. However, when pilot roll inputs become rapid, high magnitude and rapidly reversed, lateral control forces decrease due to intermittent breakout of the lateral control system jam over-ride mechanisms. This breakout is caused by exceeding the aileron actuator rate limit described in (d) above.
In summary, the combination of the above characteristics has the potential to contribute to unintended roll oscillations. In particular, a series of rapid, high magnitude roll inputs, rapidly reversed, can cause non-normal control wheel force. Although most flight crews will never experience these phenomena, pilots should be aware of the characteristics that can increase the potential for roll PIO.
Flight crews should be aware of the potential for pilot induced, unintended roll oscillations during final approach to a landing. The potential is increased in gusty wind conditions. Unintended roll oscillations may develop as a result of high rate, high magnitude, rapidly reversed control wheel inputs.
If either pilot suspects a PIO event, they should announce the situation. Flight crews experiencing untended roll oscillations should immediately stop control wheel input and allow the roll attitude to stabilise. If the aircraft becomes stabilised in a position from which a safe landing can be made, the landing may be continued. A go-around should be initiated if oscillations do not diminish or, if at any time the aircraft is not in a position from which a safe landing can be made.
Boeing Flight Operations Technical Bulletin 757-69, 2nd August 2002