Approach mode vs GPSS on AP

WannFly

WannFly

Final Approach
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Priyo
I have a button on my AP - APR mode that I haven't touched. from what I read in the manual, it says, at IAF, hit NAV and then APR , sit back, relax and enjoy the rest of the flight to minimums.. or something to that effect.

now after I installed the G5 gizmo, I tried the GPSS - out from the practice area, we came to IAF for an RNAV approach, went to minimums, went missed, shoved the throttle in, GPSS took us to the published HOLD, entered the hold and kept circling around - the APR mode was never selected.

however, in the AP manual, it also says, they recommend switching to APR mode from GPSS on IAF (or wherever you are entering the approach, I am not IR rated so pardon my ignorance), then fly the rest on APR mode.

what I am trying to figure out is whats the difference if both are doing the same thing?
 
APR should switch to greater sensitivity to nav deviations. In either the operations manual or the maintenance manual the sensitivity of the circuits should be described in some detail. APR prolly arms glideslope capture for an ILS.

On the older Century autopilots, the LOC mode invoked the greater sensitivity and armed glideslope capture if it had a pitch servo and glideslope coupler.
 
bnt83 said:
The GPS probably does the same thing, my GTN shows Enroute and Terminal modes.
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I guess that can be considered similar in the sense that in both cases, it deals with increased sensitivity. In GPS with approach capability, the transition from Enroute to Terminal to Approach modes is automatic based on distances, and has to do with scaling of full CDI deflection. The autopilot modes aren't dependent on GPS.

What GPSS does, in essence, is tie the two together. It gives your GPS control over the autopilot's heading bug, but, as has been said, does not affect the autopilot's vertical modes, so no GS capture,
 
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kgruber said:
GPSS mode will not capture the glideslope. But it does a great job of flying the ILS centerline................and it is easy to keep the airplane on GS with throttle.
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Most of the current GPS such as the GNS/W or GTN series do not provide the roll steering guidance needed by a GPSS converter when inside the FAF. So if you have been doing this, here is a hint, without the roll steering signal from the GPS on an ILS, the GPSS commands wings level and will flash the GPSS annunciator. I have sat right seat and watched this happen with the pilot fat dumb and happy, totally unaware that the GPSS was no longer guiding the aircraft.
 
John, Yes. I stand corrected.

On GPSS the autopilot will faithfully follow the final approach course on a RNAV (GPS) approach.
 
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