Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I am wondering how the engine power is controlled if Auto-pilot in a coupled approach mode with VNAV capability, and the aircraft is not equipped with Auto-Throttle?
Is the pilot continuously adjusting thrust levers (or power levers in the case of turbo prop) to maintain appropriate angle of attack, as the speed, decent rate and heading is controlled by the auto-pilot...?!!
Or...there is no such thing as auto-pilot with VNAV capability and no Auto-Throttle.
There are three jobs an auto pilot/auto thrust (or auto throttle) must perform:
Without an auto throttle/auto thrust, we remove item #3.
Another way to look at it, which is quite un intuitive is to understand the various possibilities of pitch vs throttle usage.
During climb, typically we want thrust set at a fixed maximum safe level and pitch is changed to sustain an optimal (or ATC mandated) IAS/Mach speed.
For that purpose, FADEC is enough to keep the throttle at that max safe level. During optimal descents, we want the thrust levels at flight idle, and pitch is again used to maintain an optimal IAS/Mach speed.
During cruise we can either be thrust limited or airspeed limited. If there's enough excess thrust to keep airspeed, the thrust levels will be finely controlled to achieve a certain airspeed, or if there's a small thrust shortfall, max safe thrust levels are kept, and airspeed fluctuates with turbulence/changes in temperature and other factors that either change real thrust output or little pitch changes are needed to maintain altitude.
During approach what we often have is seeking a certain speed and a certain descent ramp (resulting in a required vertical speed), which requires thrust to the nearly always changing to achieve all of those two targets simultaneously. So an aircraft without auto throttle will require constant little thrust changes, but the actual vertical profile can be kept by the auto pilot, just respond to airspeed changes.
With modern EFIS, there's the airspeed tape that has a 10 second trend tape. This tells you where the airspeed is predicted to be 10 seconds from now if nothing else changes. So as long as airspeed is close enough to target speeds, just add thrust if the trend line is negative (until it zeros) and reduce thrust if the trend line is positive (until it zeros). If airspeed is a few knots off, add just enough thrust that the trend line gets back to your target airspeed just about 10 seconds from now (or perhaps have the trend line be twice as much as the deviation so you get be back to normal 5 seconds from now). That's actually quite easy to do with the trend line.
Autothrottle is quite important if you need to keep a precise thrust level to finely optimize fuel consumption. On smaller private jets that's usually not critical.
So the auto pilot does fly the VNAV approach or VNAV profile, without an auto thrust/auto throttle, your job is to control the airspeed, which is usually far easier with the trend line.
A related useful information is that on the top of the PFD (primary flight display, the one with the horizontal attitude, airspeed+altitude tapes) if you look on the top, in aircraft with auto throttle/thrust you will see 3 columns:
The left one shows what the auto thrust is doing (idle/thrust reference/air speed/...)
The center one shows the lateral navigation (heading/course/lnav/localizer/rollout/...)
The right one shows pitch control (altitude holding, vertical speed, speed, vnav, glideslope, flare, ...)
On aircraft with EFIS but no auto throttle, I think the left column is still there, just to tell the pilot what to do with the throttles, like a nice teacher.
Is the pilot continuously adjusting thrust levers (or power levers in the case of turbo prop) to maintain appropriate angle of attack, as the speed, decent rate and heading is controlled by the auto-pilot...?!!
In 747-100/200 operations at the two 747 carriers I flew for, if you chose to have the autopilot fly a coupled ILS approach and didn't have auto-throttle capability, the autopilot tracked the glideslope by changing the pitch of the aircraft.
Personally, I never did this because I preferred to hand fly all approaches, but I watched others do it when I was the non-flying pilot, and they did indeed continually adjust the thrust levers as necessary to get the desired speed, just as we did when hand flying the approach.
In the 747 classics this wasn't hard. Initially you set the thrust levers to a fuel flow of 5,000 lbs/hr per engine, maybe a little less if very light, maybe a little more if heavy. Often that initial setting would give you the speed you wanted within a few knots and no further adjustment of the thrust levers would be necessary until starting into the flare. If it didn't, you adjusted as necessary, and with experience it took very little jockeying of the thrust levers.
