Below GS on ILS

[poadeleted20]

Here's a related question, let's say you're flying a coupled approach with a glideslope enabled autopilot. How do you set your power? Wait for it to stabilize on the glideslope then set for airspeed?

in a "typical" light plane, I reduce power to the approximate precision descent setting as the nose pitches over. If I wait for it to stabilize on GS before reducing power, airspeed will run away.

 

[poadeleted20]

When do you guys disconnect the autopilot? Minimums? When you have visual?

When I decide to land.

 

[TMetzinger]

When do you guys disconnect the autopilot? Minimums? When you have visual?

Depends. The autopilot has limitations in the AFM or supplement, so it definitely gets disconnected before then.

Sometimes, if I'm practicing using the autopilot, I'll leave it coupled all the way to the limitations, even if I'm in clear weather by then. Other times, I'll disconnect as soon as I'm visual. Sometimes I'll disconnect when I'm still in IMC, because my workload is light enough at that point I'm ready to fly.

Aside from the limitations, the answer is really "I disconnect the autopilot when I'm ready/willing/wanting to hand-fly the airplane".

 

[Ghery]

This sure spiraled from the initial question. To that one, if beginning to drop below the GS I add a touch of power. I'm already trimmed for my desired air speed. Seems to work just fine.

What is this autopilot thing of which you speak?

 

[denverpilot]

in a "typical" light plane, I reduce power to the approximate precision descent setting as the nose pitches over. If I wait for it to stabilize on GS before reducing power, airspeed will run away.

Ron, some might benefit from knowing what a "precision descent setting" is, and how one gets them for their aircraft.

(I know. I'm just saying no one said anything about it, and I'm sure there's folks reading who glossed over it, and wouldn't know why you'd want such a thing... which would be... exactly this scenario... GRIN!)

 

[Ranger]

I use power for descent rate, yoke (then trim) for airspeed on a GS.

Disregard everything else you've heard on this thread, except this. Ghery seems to be in the know as well.

 

[Ghery]

Disregard everything else you've heard on this thread, except this. Ghery seems to be in the know as well.

Thanks. I just passed my IR ride a couple weeks ago, so it's still fresh.

 

[poadeleted20]

Ron, some might benefit from knowing what a "precision descent setting" is, and how one gets them for their aircraft.

For those unfamiliar with the "control and performance" method of instrument flying as taught and written by the late Peter Dogan, founder of Professional Instrument Courses and author of The Instrument Flight Training Manual (widely available from internet aviation book sources)...

You can determine experimentally the pitch and power settings for the six basic phases of instrument flight, i.e., Climb, Cruise, Cruise descent, Approach Level, Precision Descent, and Nonprecision Descent, with the desired configuration for each phase (e.g., approach flaps and gear down for Approach Level). Then, when you want that particular performance, you establish the configuration, pitch, and power for that phase, and you should get (or at least get very close to) the desired performance (e.g., 500 ft/min and 90 KIAS for precision descent, if that's what you need for your plane). After that, you make whatever small adjustments are needed to pitch and power to obtain the exact performance.

So, if I'm in my Tiger, flying along at 90 knots approaching the glide path in approach configuration (which for me is 1/3 flaps and gear "down and welded"), as the GS needle starts to drop to center, I just reduce power 400 RPM, the nose will drop about 3 degrees (to seek trimmed speed), and I'll be pretty close to tracking the GS needle. See Dogan's book for more on the subject.

 

[dmccormack]

Another excellent resource is Gene Hudson's Instrument Flying Made Easy.

Two key take-aways:

1. Figure out the power, attitude, configuration combinations and fly those.
2. Only scan the instruments that will give you relevant information for the current maneuver/state

 

[Rndmtrvlr]

Yet another book that describes this technique in a fair amount of detail and even gives "base" numbers for a variety of aircraft is "Positive Flying" by Richard Taylor and William Guinther.

I believe it's out of print, but can be found on Amazon from some resellers. I've also found a couple of copies at Half Price Books on occasion.

 

[Rex Kramer]

Here's a related question, let's say you're flying a coupled approach with a glideslope enabled autopilot. How do you set your power? Wait for it to stabilize on the glideslope then set for airspeed?

I reduce the power as necessary to maintain speed as the pitch decreases to follow the glideslope. In theory, I have done this before so I know what target power setting I'm looking for.

However, in most cases I'm not fully configured for landing when intercepting the GS. I usually have partial flaps in. Partial flaps and an idle power setting will give me my desired airspeed. When it's time to slow up, drop the gear and add final flaps with power at idle until at final approach speed, then increase power to the target power setting for that config, adjusting as necessary for environmental variations, namely how strong the headwind/tailwind is as it affects your final approach power setting.

 

[jesse]

For those unfamiliar with the "control and performance" method of instrument flying as taught and written by the late Peter Dogan, founder of Professional Instrument Courses and author of The Instrument Flight Training Manual (widely available from internet aviation book sources)...

You can determine experimentally the pitch and power settings for the six basic phases of instrument flight, i.e., Climb, Cruise, Cruise descent, Approach Level, Precision Descent, and Nonprecision Descent, with the desired configuration for each phase (e.g., approach flaps and gear down for Approach Level). Then, when you want that particular performance, you establish the configuration, pitch, and power for that phase, and you should get (or at least get very close to) the desired performance (e.g., 500 ft/min and 90 KIAS for precision descent, if that's what you need for your plane). After that, you make whatever small adjustments are needed to pitch and power to obtain the exact performance.

So, if I'm in my Tiger, flying along at 90 knots approaching the glide path in approach configuration (which for me is 1/3 flaps and gear "down and welded"), as the GS needle starts to drop to center, I just reduce power 400 RPM, the nose will drop about 3 degrees (to seek trimmed speed), and I'll be pretty close to tracking the GS needle. See Dogan's book for more on the subject.

My training is based on that very same concept and I don't even consider moving a student onto approaches until all of that is nailed along with a list of other things.

 

[TMetzinger]

My training is based on that very same concept and I don't even consider moving a student onto approaches until all of that is nailed along with a list of other things.

I don't fly an airplane under IFR unless I've determined the "pitch-power-performance" for those six phases, at a minimum.

When I've got some more time in the airplane I can open up the envelope for more options (i.e. 140KIAS on the precision approach in a Mooney) to accomodate non-normal situations. But all that gets done in VMC first.

I plan to tell students that they need to figure these numbers out for themselves, and that they can pretend they're a test pilot while they do it.

 

[godfreypilot]

My CFII was an engineer, and he has an interesting perspective on the pitch/power dilemma on an ILS glideslope. In his words:

Let's assume that you're flying a glideslope during which all controls except the throttle stay constant (flap/trim settings do not change during the descent).

In this case, your power setting directly and exclusively affects your descent angle (airspeed/trim has no effect). For example, trimming the nose up (or applying back pressure to the yoke) decreases your sink rate, but at the expense of also decreasing your forward speed. The result is that, ignoring some aerodynamic effects that change with your speed, your OVERALL descent angle should remain largely unchanged.

On the approach, aircraft experience wind gusts that will blow them off glideslope (this is why its hard to fly a perfect ILS!). But unless these gusts persist for longer than a few seconds (such as when flying into a new wind layer), they do not significantly alter your descent angle. A strong enough change in wind could blow you a degree or so below glideslope, but that doesn't necessarily mean that your overall descent angle has changed.

For this reason, I've found the best way to fly an ILS is to set my power and trim for my target airspeed at the glideslope intercept, and then use the YOKE to keep the plane on glideslope (in other words, I am flying the CDI, not the airspeed indicator). I will only change my power setting if my airspeed changes and then stays changed for a period of time as a result of my yoke work (this would indicate a new wind layer). The trim stays put throughout the approach, since changing it disrupts the power/descent angle relationship.

I don't think that this is the definitive "right" answer, but it works for me so I thought I would share.

Happy flying!

 

[bobmrg]

My CFII was an engineer, and he has an interesting perspective on the pitch/power dilemma on an ILS glideslope. In his words:

Let's assume that you're flying a glideslope during which all controls except the throttle stay constant (flap/trim settings do not change during the descent).

In this case, your power setting directly and exclusively affects your descent angle (airspeed/trim has no effect). For example, trimming the nose up (or applying back pressure to the yoke) decreases your sink rate, but at the expense of also decreasing your forward speed. The result is that, ignoring some aerodynamic effects that change with your speed, your OVERALL descent angle should remain largely unchanged.

On the approach, aircraft experience wind gusts that will blow them off glideslope (this is why its hard to fly a perfect ILS!). But unless these gusts persist for longer than a few seconds (such as when flying into a new wind layer), they do not significantly alter your descent angle. A strong enough change in wind could blow you a degree or so below glideslope, but that doesn't necessarily mean that your overall descent angle has changed.

For this reason, I've found the best way to fly an ILS is to set my power and trim for my target airspeed at the glideslope intercept, and then use the YOKE to keep the plane on glideslope (in other words, I am flying the CDI, not the airspeed indicator). I will only change my power setting if my airspeed changes and then stays changed for a period of time as a result of my yoke work (this would indicate a new wind layer). The trim stays put throughout the approach, since changing it disrupts the power/descent angle relationship.

I don't think that this is the definitive "right" answer, but it works for me so I thought I would share.

Happy flying!

When I was checking out in the Citation II, my ex-USAF fighter pilot instructor said "Pitch to the glideslope, power to the airspeed." I think that is what you are saying.

Bob Gardner

 

[jwriteclub]

Briefly back to the autopilot question:

Barring any limitations on the usage of the autopilot, keep it connected until you transition to visual flight.

There's a non-trivial chance that you're going missed. At which point, you'll be applying power, cleaning up the plane, flying the missed and (eventually) talking to ATC. If you have a navigator that can sequence to the missed, or else have the initial missed course set in the AP, then you can easily sequence onto the missed.

For the specific example of the DA-40, where (IIRC) you can use the KAP-140 down to ILS minimums, you need to be careful of the vertical speed setting you have set, as the KAP-140 will happily pull you into a stall to get to the target VS for the missed. In a situation like this, I'd typically (assuming I'm not on a GPS approach where I need the navigator for the missed), engage only heading mode on the AP as soon as hitting missed (remember setting your heading bug to the missed approach course). Manually fly the climb until power is stabilized and the airplane is clean. Then engage the VS mode on the AP (which you also preset to your expected missed approach VS). Finally, engage the nav mode on the AP to fly the missed.

The key to all of this is know how the system handles going missed. It's crucial to fly various missed approaches with the AP coupled to learn the procedure. For instance, do you have to push a button on the navigator to engage the missed? Does it sequence automatically when you reach the MAP? If you're not familiar with how the AP will react on the missed you can find yourself in a bad attitude quickly.

~ Christopher

 

[John Collins]

In this case, your power setting directly and exclusively affects your descent angle (airspeed/trim has no effect). For example, trimming the nose up (or applying back pressure to the yoke) decreases your sink rate, but at the expense of also decreasing your forward speed. The result is that, ignoring some aerodynamic effects that change with your speed, your OVERALL descent angle should remain largely unchanged.

I am not sure I would agree with your instructor's analysis. Pitch changes have a greater effect on changes in vertical speed and the descent angle than they have on speed.

On the approach, aircraft experience wind gusts that will blow them off glideslope (this is why its hard to fly a perfect ILS!). But unless these gusts persist for longer than a few seconds (such as when flying into a new wind layer), they do not significantly alter your descent angle. A strong enough change in wind could blow you a degree or so below glideslope, but that doesn't necessarily mean that your overall descent angle has changed.

In my experience, the effect that wind gusts have on pitch attitude has more to do with deviations from the GS. Just like bracketing the heading to hold a lateral course, bracketing the attitude indicator pitch angle and monitoring the VS are important ingredients in flying a GS. Long before you see a deviation on the GS, you will see a VS deviation and a pitch deviation. One technique I use is to cancel these deviations out, so if my pitch went to higher than required along with a lower VSI indication for several seconds, I would compensate by pitching to a slightly lower attitude than my target for roughly an equivalent amount of time. A typical GS vertical CDI Full Scale Deflection (FSD) is +/- .7 degrees, so a 1 degree deviation would be well above a full fly up indication and would demand a missed approach.

For this reason, I've found the best way to fly an ILS is to set my power and trim for my target airspeed at the glideslope intercept, and then use the YOKE to keep the plane on glideslope (in other words, I am flying the CDI, not the airspeed indicator). I will only change my power setting if my airspeed changes and then stays changed for a period of time as a result of my yoke work (this would indicate a new wind layer). The trim stays put throughout the approach, since changing it disrupts the power/descent angle relationship.

I would also not recommend flying the airspeed as it should remain relatively constant (within 5 to 10 Knots) unless you make a power change. I use a target pitch attitude and vertical speed to control the vertical thru the yoke and do not chase the vertical CDI indication. I will make a trim change if the CDI deviations are all in the same direction.

The autopilot flies the GS by controlling the pitch axis and does not have a power input, so the airspeed is whatever it is, unless the pilot makes a power adjustment.

 

[midlifeflyer]

There's a non-trivial chance that you're going missed. At which point, you'll be applying power, cleaning up the plane, flying the missed and (eventually) talking to ATC. If you have a navigator that can sequence to the missed, or else have the initial missed course set in the AP, then you can easily sequence onto the missed.

An important thing to remember is the reason why GPS units don't automatically sequence to the missed.

If you sequence to the missed, the autopilot will fly there. Directly. But the missed for the procedure might include a straight ahead or turning climb before heading to the missed holding fix. You want to makes sure you do that climb before you tell the autopilot to fly there.

 

[John Collins]

An important thing to remember is the reason why GPS units don't automatically sequence to the missed.

If you sequence to the missed, the autopilot will fly there. Directly. But the missed for the procedure might include a straight ahead or turning climb before heading to the missed holding fix. You want to makes sure you do that climb before you tell the autopilot to fly there.

Good point on why to not have the GPS sequence automatically to the miss after the MAP, but prior to the most recent update of the TSO for WAAS GPS units, it was required to auto sequence. The GNS480 auto sequences, the GNS430W/530W does not and has an FAA approved deviation from the requirement. Depending on the implementation, the G1000 will auto sequence.

 

[midlifeflyer]

Good point on why to not have the GPS sequence automatically to the miss after the MAP, but prior to the most recent update of the TSO for WAAS GPS units, it was required to auto sequence. The GNS480 auto sequences, the GNS430W/530W does not and has an FAA approved deviation from the requirement. Depending on the implementation, the G1000 will auto sequence.

Interesting. Another good reason not to follow the GPS blindly.

 

[TMetzinger]

Does it auto-sequence, or does it have the next waypoint tee'd up but not ACTIVE until you hit a key.

I believe that it may vary on the procedure too - I vaguely recall the WAAS G1000 auto-sequencing for:
MAHPs straight ahead
Missed procedure coded fully (so that the turn or altitude constraint is modelled)

and going into SUSP for other missed procedures.