Hey guys it's my first post here. Maybe you can help but I want to know why sometimes the approach function of the autopilot doesn't do its job. Although I have the ILS keyed in the radio and the NAV/GPS switch set to NAV, sometimes the approach or nav button on the ap doesnt fly the approach. Any idea why?

Welcome to Avsim.

 

Does this happen at a special airport or does it occur randomly all over the place? Perhaps give an example where it didn't work, so others can check what could be the cause.

 

Which plane are you flying there?

It last happenned today in my Carenado F33A. It has happenned before in my PA A320 and even the default jets I think.

My extensive testing of the ILS and autopilot approach function indicate it is temperamental, finicky, and just plain unpredictable.

 

I have flown thousands of ILS approaches at dozens of airports in about 30 different airplanes ranging from fast jets to slow GA. At times I have been exactly on the Glideslope and within one dot horizontal of the ILS course and have a good NAV1 signal as indicated by the NAV1 ident tones. I press the Autopilot APR button and the plane either dives or climbs at a crazy angle. Sometimes the autopilot recaptures the glideslope - other times I have to manually land.

 

Other times, under what seem like identical conditions, the autopilot/approach function works perfectly.

 

I have SAVED Flights that are exactly on the glideslope with no horizontal deviation at the OuterMarker beacon. I SAVE the flight before I turn on the AP and APR. Eight out of ten times when I fly the saved approach and turn on the AP and APR the ILS autopilot captures the GS and takes the plane in for a nice landing. Two out of ten times the GS is not captured and the plane either dives, climbs, or flies straight ahead. All from exactly the same spot!

 

What I do know is that the plane needs to be within 1/2 bar vertical deviation or less and usually low so that the plane flies into the glideslope from below. The horizontal deviation does not seem to be as critical. You also need to be within the ILS arrow shown on the FSX map (I think that means you need to be between the IAF and the TDZ).

 

ILS autopilot landings have been a never ending source of frustration to me for many years and continue to be so.

 

Most of the time ILS autopilot approaches work perfectly other times not so.

 

AND... there are many airports where I can not get the ILS approach to work even if I am exactly on the glideslope with no (zero) horizontal deviation. This usually occurs when there is an ILS approach from both ends of the runway.

Is the AP not capturing the localizer, the glide slope, or both?

 

You can miss the glide slope if you're too high, the glide slope must be captured from below. The localizer will not capture if you're too far off the localizer course.

 

I usually first make sure the CDI starts to move, then switch to NAV mode, then when the glide slope indicator starts to move I switch to APP.

 

If you're flying IFR and following ATC vectors, you'll generally be lined up properly. If not, then it may help to follow the ILS approach chart or the approach procedure in the GPS if there is one. If there's no procedure, try to approach on the runway heading at around 2000 AGL.

the glide slope must be captured from below.

 

This.

 

Ever since I understood this, capturing the glideslope with the autopilot has worked for me every time.

 

Welcome to the Forum.

As stated, you must be below GS, with the needle up, in order to capyure it.

 

You don't need to use the NAV button. It is used to track a VOR radial. The vertical needle needs to be active and off the peg to capture the LOC.

 

Dave