Rudder ratio

Two minutes after takeoff, at 2344 25, the captain noted computer alarms mach speed trim and rudder ratio. The meaning of these alarms was not known to the aew... 

The aircraft hit the sea 20 km from the Puerto Plata at 2347 17, and all on board were killed. The entire flight had lasted less than 5 min, and it had been less than three minutes since the alarms mach speed trim and rudder ratio had been received. 

The US National Transportation Safety Board (NTSB) issued various Safety Recommendations on May 31, 1996. These included a recommendation that the Boeing 757 flight manual should be revised to notify pilots that Simultaneous activation of the mach speed trim and rudder ratio advisories is an indication of an airspeed discrepancy. The NTSB also required Boeing to modify the alarm system to include a caution alert when an erroneous airspeed indication is selected. Various other changes to the flight manual were also instructed. Simulator training... 

Accident reports, with perfect 20-20 hindsight, often blame the pilot-operator. There were less than 3 minutes between receipt of the incomprehensible mach speed ratio and rudder trim alarms, and the crash into the sea. This accident was caused by poor design of the human-machine interface, which was then compounded by pilot errors - and not the other way round. 

The crew were unaware of the meaning of the mach speed ratio and rudder trim alarms, because they had never seen the National Transport Safety Board (NTSB) Safety Recommendations from the Birgenair 301 accident. The report into the Aeroperu accident noted pithily that the NTSB Safety Recommendations were not distributed with the necessary urgency. ... 

The effect of water depth to draft ratio and influence of rudder angle examined on big physical model with geometrical scale 1 16 was presented in (Abramowicz-Gerigk, 2010 Abramowicz-Gerigk, 2011). The mean axial velocities v of propeller jet behind the rudder in deep water (water depth to ship draft ratio h/T = 3) and shallow water (h/T = 1.2) conditions related to —jet velocity in deep water with rudder angle 0 are presented in Table 1. 

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