FLY 'EM BETTER -- Coupled ILS Approach Tips
By Jim IrwinMost of us with IFR time under our belt and ILS approaches into tight IMC weather conditions in our logbook, have encountered challenges with coupled approaches. On more than a few occasions the approach controllers of the world have attempted to fit a General Aviation airplane between 2 Air Carrier aircraft by vectoring us directly to the outer marker. While this can be difficult in 1000' & 3 miles weather, it makes the approach almost impossible when the weather is really low.
If all this occurs after being held well above the approach altitude until very close to the airport demanding a rapid descent to the marker, the chances of getting the autopilot to fly a good coupled approach are slim.
Here we hope to give you a few tips on how to make flying a coupled approach work a little better. However, these are tips only and you should be completely familiar with the exact procedures outlined in The Airplane Flight Manual Supplement for your aircraft and autopilot as well as with the Pilot Operating Handbook for your system.
From your initial IFR training you'll remember your instructor stressing how being prepared is critical to flying a good approach. He told you to read the approach procedure thoroughly before you got close to the airport by tuning ATIS early, thereby learning which approach was in use. For ILS approaches he told you needed to know the Approach Altitude, the ILS inbound course, the DH and the Missed Approach Procedure as well as the other data found on your Approach Plate. You learned to get the aircraft trimmed, in the proper configuration and at the proper airspeed for the approach well in advance of intercepting the Localizer and Glideslope.
Asking the autopilot to fly the approach requires some help from you, since like you, it will do a better job of flying the approach if it is well prepared. Autopilots in general, and S-TEC autopilots specifically, do not require any special procedures. They will, however, do a better job for the pilot who understands what they need.
First, you need to understand that an autopilot will not make an adjustment until it sees a need for a change, i.e. a deflected needle indicating the aircraft is off course, or sensing a loss of altitude before initiating a climb to get back to the coupled altitude. The autopilot computer will however, anticipate certain conditions such as the need to shallow the intercept angle as a reaction to seeing the rate of change in the needle deflection.
Second, as mentioned earlier, getting the aircraft trimmed and configured for the approach well in advance is important. The autopilot also needs time to adjust to the changing configurations of the aircraft as the approach progresses. If the system has the aircraft trimmed and ready for the approach it will do a good job of flying the airplane to minimums. If it's busy trimming and interpreting rapidly changing needles and power changes, all at the same time, it may not do a good coupled approach.
Third, you need to know the needle deflection values which must be met for LOC & GS intercept. These can be found in the POH and The Flight Manual Supplement. For most S-TEC systems the Localizer Needle should be at a deflection equal to or less than 50% of total needle deflection and the GS needle should show a 60% of full needle deflection up or more, as you engage the autopilot for the coupled approach as outlined in the POH. If you are not within either of these values the autopilot will not couple requiring a manual coupling procedure. The manual coupling procedure is also outlined in your POH.
Fourth, all power setting changes should be made gradually. In a constant speed propeller equipped aircraft, 1” of manifold pressure at a time. This again gives the autopilot time to trim to the changing configuration.
So what should you do as you prepare to ask the autopilot to fly a coupled ILS Approach? Get everything ready well in advance of the intercept including getting the airplane to the approach altitude as quickly as possible. That way, after you engage Altitude Hold it will have time to retrim the aircraft. Sometimes you have no control of when they'll let you down to the approach altitude, but when you do, get down as early as possible. Set the autopilot up for ILS intercept equally as early so the needle deflection values are met, namely less than 50% of full needle deflection off the LOC centerline and under the GS by more than 60% of full needle deflection.
At some point before you intercept the GS you should put the flaps to the approach flap position or as specified in the POH, again to give the autopilot time to trim the airplane before the marker. Keeping track of your position in the approach using crossing radials and/or ILS DME can be very helpful. At the marker drop the gear and retard the throttle to a preselected power setting that works for your airplane. After GS intercept make small power adjustments to control the speed.
One last note, in the recent past the FAA has issued some restrictions as to how low a coupled approach can proceed based on the specific aircraft in question. This restriction will show in your Flight Manual Supplement if it applies to the aircraft you're flying. In these cases a 200' DH Category I Approach may only be flown coupled to some point higher than 200' AGL.
Remember, be prepared, anticipate, read your manuals, and allow the autopilot time when trimming for configurations changes. As one last suggestion, we recommend that you do several practice approaches in VFR weather to rehearse these procedures and to learn what power settings work - then, enjoy flying accurate coupled approaches.
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