Helicopter missing in the Med

[JohnDixson]

Good post, and I look forward to the response from the RVguy. You two are clearing the air a bit re FBW, and there was one issue touched on that has been a problem to me for decades-the “out of dateness“ of the longitudinal static stability requirement.

The idea behind the requirement sounds fine: it should require forward control input to go faster and vice versa. The associated “ stability “ part-that the stick position vs speed must show a positive/negative slope sounds fine, until one thinks about it. The assumption is that if the pilot doesn’t move the control, the aircraft will stay right there, on speed. Its stable..All is well. The reality is that when a change in speed is demanded, the control is moved forward ( for example ) then as the ship gets to the desired speed, the pilot makes the adjustments to stop the acceleration and fine trim the ship to the new speed. Its “ nice” if that measured position is forward of the original, but what matters is if the ship stays there.

In the modern fleets of machines there are some that stand out as having to add equipment ( sensors, actuators etc ) to meet the positive stick position slope: the CH-47 series, the UH-60, the S-76 and the 53E. DASH, Pitch Bias Actuators ( PBA ) etc. The UH-60 case stands out in my mind. There was a speed area at max aft CG where the stick position measurement in smooth air etc was flat to slightly negative. We had to add all the claptrap associated with the PBA. We tried to reason with the Army that the pilot would never know the difference ( if he didn’t have the PBA )because the AFCS had airspeed hold automatically locking on after 12 seconds ( i.e., the ship would stay where the pilot put it, speed wise ). Lost that argument. Scene shifts forward 3-4 years and the PBA subsystem was a frequent cause of maintenance actions. Had another meeting with the Army, who now knew that the pilots couldn’t determine any change in the handling and the PBA was removed.

My point is that if the aircraft stays where the pilot trims it, the stick position slope argument with modern control systems is baseless-the modern control system solves the original intent with modern technology. Solution on the other side of the discussion is to modernize the regulations.