Recommendation from a 1980 test flight accident:
Incorporate the following information into the DC-9-80 training manuals
and training programs under the flight control and landing sections:
When thrust reversers (located just forward of the vertical stabilizer) are used during landing rollout, the exhaust gases from the engines are deflected by the thrust reverser buckets in such a manner that the free-stream airflow over the vertical stabilizer and rudder is blocked, reducing the
effectiveness of these surfaces. At a nominal airspeed of 100 KIAS, the reduction in rudder effectiveness with increasing symmetric reverse thrust levels is shown below.
Engine Thrust Setting Max Rudder Effectiveness available(%)
Forward Idle 100
Reverse Idle 65
1.3 EPR (Reverse) 25
1.6 EPR (Reverse) minimal
Rudder effectiveness also decreases with decreasing airspeed. On a dry runway, directional control is easily maintained by differential antiskid braking and nosewheel steering.
However, under adverse conditions such as a slippery runway with rain, snow, or ice, when crosswinds reduce the braking effectiveness of the gear on the upwind wing, or when a high speed landing is made with both hydraulics systems out (i.e., flaps/slats retracted, ground spoilers, rudder hydraulic boost, nosewheel steering all rendered inoperative, and brake
antiskid systems limited by hydraulic accumulator pressure), the vertical stabilizer and rudder will be the primary source of directional stability and control during the high speed portion of the landing rollout. Under these conditions, it is important to make allowance for the adverse effects of reverse thrust on the effectiveness of the vertical stabilizer and rudder.
http://www.fss.aero/accident-reports...0-06-19-US.pdf