One of the recommendations arising from the National Transportation Safety Board (NTSB) USAir Flight 427 Accident Investigation was the forming of an independent team, the purpose of which would be to completely (and independently) review all aspects of the Boeing model 737 rudder system with- out regard to probability of occurrence. This team was named the Engineering Test and Evaluation Board (ETEB), and comprised engineers and pilots from the FAA, Boeing Military, USALPA, NASA, Ford Motor Company and the NTSB. The FAA was designated team leader.
After more than 12 months of activity, the ETEB handed down their report in mid-September 2000. The first two chapters are available on the World Wide Web, via the FAA site at http://www.faa.gov/apa/etebl.doc and http://www.faa.gov/apaleteb2.doc, unfortunately the remaining chapters will most probably never be publicly available due to issues arising from propriety information contained therein. The first two chapters contain the Executive Summary.
As a result of the findings as detailed in the ETEB report, the airplane manufacturer, Boeing, and the FAA have agreed to mandate a rudder enhancement program for the 737, the details of which were presented at this conference. It is important to realise that these new enhancements are in addition to the Rudder Pressure Reducer (RPR) and Digital Yaw Damper (DYD) fittment program currently underway.
These consisted of training issues (understanding of rudder control system, effects of aft-column on upset recovery, effects of blowdown, crossover speed, mechanical and hydraulic redundancy, and simulator limitations/training), maintenance program enhancements and rudder system redesign.
Flight Crew procedural changes
To begin with, the Jammed or Restricted Rudder Procedure has been clarified and shortened in the light of simulator trials (using 10 crews) conducted during the ETEB investigation. It was observed that crews found the old procedure confusing and difficult to complete. Consequently, the procedure has been renamed (Uncommanded Rudder), simplified and no longer requires manual reversion as a possible configuration dictated by the checklist. Inappropriate use of this new checklist has been proven not to worsen the situation. This new checklist is initially similar to the Uncommanded Yaw or Roll checklist, but consists of only the first two items as memory (recall) requirements, relying upon the Digital Yaw Damper self test function to initiate an auto shutdown of a malfunctioning channel. The Yaw Damper has been retained as a recall item in the Uncommanded Yaw or Roll checklist in order to protect aircraft not yet fitted with the RPR and DYD. The new procedure also allows a landing in the normal flap configuration, since there is no aerodynamic reason to use lower flap settings, as was required by the old checklist.
With this new procedure, there are two possible checklists available to crews to use in the event of a rudder anomaly and subsequent diagnosis – this being less than ideal. One checklist should exist for such a situation preventing confusion and the loss of valuable time. This matter was raised by an operator at the conference, with Boeing promising to explore this possibility.
Rudder Control System Design Enhancement
The new system will be functionally equivalent to a three-actuator system, but using two Power Control Units (PCU’s). Such a system implies independent actuators, servo valves, valve inputs and hydraulic systems (A, B and Standby). A malfunction in one actuator is reacted to by a second actuator, as is the case with other Boeing models such as the 757 and 767. Simplex valves (as used in the 757/767) will be incorporated, eliminating the need for the current dual-concentric design. Each valve is commanded with a dedicated control rod containing a mechanical override device, with the main PCU incorporating a sensor to indicate when a malfunction (i.e. jam or break) occurs. No new technology will be used, and all have proven themselves in service - we do not want to make the cure worst than the disease.
This will eliminate the need for unique crew procedures (i.e. Uncommanded Rudder checklist) for the 737, and will affect all 737 models from the -100 through to the -900 (currently in flight test). It also addresses the issue of reliable redundancy.
The design phase of this new system is almost complete, with first flight expected in February 2003, certified by June of the same year – first kits should be available by September 2003. Entire U.S. fleet conversion is expected to be completed sometime in 2006, with Boeing paying for "parts and paper" – paper involving certification, service bulletins and maintenance instructions.
Additional ETEB findings
The ETEB also recommended fittment of a rudder position indicator and hydraulic isolation switch to quickly and with a single action, isolate the rudder completely. However, the FAA has never certified a flight control position indicator system to be used during an upset recovery, and maintained that such an indicator was not required (even though the ETEB recommended it be used as a guide for diagnostic purposes alone). Additionally, the FAA found that this new system would be to such a level of redundancy as to not require an additional "kill" switch (apart from adding complexity).
An important comment incorporated into the ETEB report related to the limitations of present simulators to adequately train crews in rudder anomalies – specifically the aerodynamic performance limitations of simulators that do not include recent upgrade packages which improve simulator handling characteristics during recovery/event training. These upgrades have been available for over a year, and indeed have recently be upgraded, however, since this area is not covered by certification requirements, and is thus not mandatory, airlines are reluctant (in general) to purchase such packages (often requiring hardware upgrades as well).
Until the world fleet is completely modified, at best guess by around 2010, there still will remain an area of exposure during take off, approach and landing, where the aircraft is below crossover speed (where rudder authority exceeds that of the lateral flight control surfaces – aileron and spoilers). This period is significantly reduced on RPR equipped aircraft, since at radio altitudes above 1000ft the hydraulic pressure to the rudder is ramped down, limiting rudder travel, and therefore effectiveness, in the event of an anomaly. Nevertheless, one U.S. operator whose fleet comprises some 200 737s, has calculated an exposure time of 120 hours per month (based upon 1200 sectors per day), which is not in itself insignificant.
The counter to this argument (certainly the Boeing position) is that the crew would have checked the rudder for correct function prior to departure, coupled with the extremely low probability of "failure" so soon after this check in this small time period, meaning that this scenario can be effectively eliminated from reasonable consideration.
In any case, one U.S. operator requested that Boeing evaluate and research the possibility of a crew procedure in the event of a rudder malfunction below 1000ft, to which Boeing has agreed. Ultimately this may end up being in the form of a training scenario emphasising correct and timely crew action (true of any such event).
This new program will certainly help to eliminate all reasonable failure modes involving the 737 rudder system, putting it in line with other Boeing models. It is important, however, to keep perspective, and be aware of the fact that it will be a long time before the world fleet will be brought up to these new requirements.
- Members should encourage their airline to accelerate, wherever possible, the fittment of RPR and DYD equipment to their aircraft (this was encouraged by Capt. Mike Carriker- 737 Project Pilot - at the 1999 ADO/2). Obviously this would also be the case for the additional enhancements discussed above, when they become available.
- Respective airlines must be encouraged to incorporate the upgraded software package in their simulators, and require crews to complete, on a reasonably regular basis, the suggested training scenarios for rudder anomalies, including extensive upset recovery training. This should include a departure scenario below 1000ft above ground level. Preferably, regulatory authorities should mandate fittment of the revised software package and training scenarios.
- IFALPA should encourage Boeing to produce one checklist for these uncommanded events since this would assist crews involved in such events, as discussed earlier.
- The importance of the pre-departure rudder check should be highlighted to crews -the low probability of failure calculation after take-off and below 1000ft is predicated on its completion (recommended to take 2-3 seconds smoothly to full travel in each direction in order to fully exercise the PCU. Rapid movement is not recommended). Also important is the reporting of unusual rudder activity or ‘kicks’. The rudder pedals should never ‘kick back’ in flight. From flight crew debriefs of the simulator sessions, the ETEB learned that none of the flight crews understood the serious nature of rudder pedal kicks or uncommanded rudder pedal movement. The yaw damper is designed to move the rudder without back driving the rudder pedals. Even with a yaw damper malfunction, the yaw damper alone will not cause pedal motion. Pedal motion is an indication of a problem, such as high friction or a jam in the standby PCU or interference in the feedback linkage in the main PCU.
- Airline training should include improved knowledge of the rudder system. This would include a revisit of rudder blowdown principles and the relationship between rudder pedal and control surface position.
- A standardized reporting format of rudder anomalies and compilation of a database should be explored by Boeing and the relevant regulatory authorities.