Approaches with autopilot restrictions

[RodneyHooverCFI]

Good luck, AB! I learned the "trick" with that approach into Livermore that gets lots of students- one CFII and my flight school ratted out the other CFII (the one who does stage checks) and warned us about the "gotcha"...

See if you can find it:
ILS Approach for Livermore KLVK

Didn't want to hijack AB's thread. Anyone know why some approaches like the one listed above have restrictions against autopilots?

 

[vettdvr]

 

[Ward Holbrook]

Good luck, AB! I learned the "trick" with that approach into Livermore that gets lots of students- one CFII and my flight school ratted out the other CFII (the one who does stage checks) and warned us about the "gotcha"...

See if you can find it:
ILS Approach for Livermore KLVK

Didn't want to hijack AB's thread. Anyone know why some approaches like the one listed above have restrictions against autopilots?

(Hand goes up at the back of the room) Oh, oh, I know this one. Pick me, pick me.

 

[Ward Holbrook]

The limitation on the approach charts requires the pilot to uncouple the autopilot at or prior to this altitude to avoid erratic autopilot performance caused by a reversal in the glideslope signal. A reversal is defined in the US Standard Flight Inspection Manual, FAA Order 8200.1B, para 217.42 (b) as a change in direction ... (which) extends for at least 1500 feet along the approach with an essentially continuous slope. The use of facilities which do not meet the change/reversal tolerance shall be limited by a NOTAM that withholds authorization for autopilot approaches below an altitude (MSL) which is 50 ft higher than the glidepath altitude at which the out-of-tolerance condition occurs.

A glideslope signal is not straight. Its signal bends and it has roughness that, if severe enough, can actually cause the airplane to climb as it approaches the runway. This is also a reason why it is very important not to couple to an autopilot to the GS outside of the precision final approach fix. This is because it has never been flight tested beyond the precision final approach fix and can be very erratic - think bicycle spokes diverging as you get further from the hub towards the rim. This type of signal can cause an autopilot to fly erratically or in the worst case, disconnect. So the FAA restricts the use of autopilots in these areas on the GS.

Does this make the that particular ILS unsafe? No, when you are hand-flying through this area you will naturally average out the deviation, whereas the autopilot wont. You don't run into these types of limitations very often, but they are out there.

 

[RodneyHooverCFI]

Hmm, that makes sense, just don't recall the reason being listed anywhere. Thanks Ward!

 

[Awkward Bird]

Yeah... thanks Ward! I hope someday to have an autopilot to deal with, and I definitely prefer to know WHY something is that way.

 

[Nightowl]

Good to know. My autopilot doesn't couple to the GS, so have to hand fly that part anyway. Given the option, that's another reason to shoot an RNAV LPV approach instead of an ILS if both are available - the needles will be much steadier with the former.

John

 

[Glenn Darr]

that is good to know. Sometimes I set my autopilot to the ILS, and just do the pitch part, especially when I have been busy with things. the a/p does a pretty good job.