Why are airliners not inherently "speed stable"? In a light aircraft, once the pilot has trimmed for a speed, the power controls the pitch (climb/descent rate).

I don't know the technical name for this stability, so I chose "speed stable" for now.

The fly-by-wire (FBW) of the 787 (for example) mimics this behavior:


  In flight, the pitch trim switches do not position the stabilizer directly, but make inputs to the [Primary Flight Computers] to change the trim reference speed. The trim reference speed is the speed at which the airplane would eventually stabilize if there were no control column inputs. Once the control column forces are trimmed to zero, the airplane maintains a constant speed with no column inputs. Thrust changes result in a relatively constant indicated airspeed climb or descent, with no trim inputs needed unless airspeed changes.—787 FCOM


I.e., once a speed is trimmed, the top sentence becomes true like in light planes.

So the related questions are:


What's the name of this stability?
Is it limited only to light aircraft?
Is it related to having a trim tab instead of a stabilizer trim? (Just a speculation.)
How can an airliner without FBW be designed to be "speed stable"?




An answer doesn't need to address the exact questions in order, I'm just communicating what I'd like to understand.



2019: The impact of the third point still confuses me:

Once the speed is trimmed (and left alone), and power is increased (for example), wouldn't having a trim tab (like on a typical light plane) have more impact on pitch response (speed stability) compared to the now-fixed stabilizer (jet-liner)? In other words, a light plane would pitch up quicker (elevator responding to the increase in airspeed), while a non-FBW jet-liner would pitch up much slower (stabilizer unaffected by airspeed) and gain more speed compared to the earlier trimmed state?
Light aircraft and transport aircraft behave exactly the same regarding trimming for speed. Trim is for angle of attack, but power affects that trim and phugoid oscillation occurs in speed and pitch, because the feedback is second order.

The Boeing FBW has no trouble mimicking anything. It simply translates the control forces and position exactly as a hydro-mechanical link would, just in electric signals. The aircraft is still stable.

Airbus aircraft are also all stable, but their FBW relaxes the stability by auto-trimming for current speed. The result is that side-stick controls flight path angle and power controls speed.

The only exception to the stability are fighter jets (starting with F-16). Those are aerodynamically unstable to get faster control response and their FBW makes them neutral (that is, the yoke/stick again controls flight path angle).