Holding the Glideslope

[Larry in TN]

If I’m trimmed at the correct airspeed in a fixed gear aircraft I can pull a couple hundred RPM off and not touch the yoke and the plane will start down the glide slope at the same airspeed.

But the pitch attitude has changed, even if you didn't do it with the yoke. You let the nose drop when the power was reduced.

If I use the yoke to make the change I still need to pull the power off or re-trim for a higher airspeed, which is an extra control input.

If you're holding the same airspeed, after a small change in power, there will be no need to re-trim. There is a new pitch-attitude and a new rate-of-descent.

If you let the nose drop on it's own, due to the power reduction, you'll get a little bit of up/down pitch while the nose seeks the new trimmed attitude. Do it yourself with the yoke, while you adjust the power, and you'll fly more precisely.

 

[dmspilot]

You can do it both ways. It boils down to technique. Your instructor is an idiot for accusing your technique of being "wrong". And as was mentioned by others, as a CFI you should be backing up techniques with written references, not "my instructor said to do it this way".

 

[chemgeek]

My approach (the way I was taught) is that the appropriate known power and pitch setting is applied to start the descent at a known airspeed. This is your coarse control. It will get you really close from the get-go. Minor, temporary excursions from the glideslope are controlled with pitch. This is your fine control. (You have already "set the numbers" for a say, general 500 fpm descent at 90 kt). If temporary excursions become persistent, say not descending fast enough, then power is adjusted to compensate for the deviation from the desired descent rate so as to maintain the desired airspeed. This sounds more complicated than it is. Power adjustments are quite slow compared to pitch adjustments, so adjusting power for every small deviation from the glideslope due to turbulence will result in one chasing the glideslope in a never-ending cycle of futility. If you chase the glideslope by constantly re-trimming only, then you may wind up significantly changing the desired approach airspeed. So, start with "the numbers" for a desired airspeed and descent rate, and make coarse adjustments with power to control general descent rate, using pitch to correct for temporary deviations. Once you figure out what is appropriate to do when, you can nail the glideslope all the way down.

 

[Martin Pauly]

Look at the physics involved. Changing pitch will affect both your airspeed and your descent angle. Sometimes that's all you need or want. At other times, you also need to change your energy level, and that of course requires a power change.

The downside I see with only preaching "pitch for airspeed, power for altitude" is that you'll find pilots adding power when they are half a dot below GS while 15 knots too fast. In that case, just pitch up a bit instead, and convert kinetic energy into potential energy. Same goes for being high and slow - just push the yoke forward, and kill two birds with one stone. All too often, I see pilots in training focus on just one of those at a time.

Once you get to the point where you are getting both low and slow, then add power. Or high and fast, then pull the throttle back.

- Martin

 

[midlifeflyer]

This one of those technique things. For me it's small changes = stick; big changes = throttle.

Its more about what I see about the human condition and the flight controls than physical science. Over the course of the years, I've noticed that, generally speaking, we have much finer control over the yoke/stick (assuming a light touch rather than a death grip) than over the throttle. Slight pressure to change pitch one or two degrees to adjust the glidepath is just physically easier and less time consuming than getting a throttle adjustment just right. I really don't care if those one or two degrees (which are probably going to change in the next 10 seconds anyway) increases or my IAS 5 KIAS in the process.

 

[Checkout_my_Six]

I trim for airspeed, adjust power for altitude, but pitching for a quick adjustment of either speed or altitude is not avoided when useful.

this....

 

[Rockymountain]

Probably depends on conditions, and challenges of the real world. Power to stay on glideslope could probably work in smooth air, VMC with low stakes for getting discombobulated, and impress an instructor next to you, if you pull it off. But in hard IMC with real potential obstructions and turbulence, much safer and more practical to stay on glideslope with pitch and control major speed excursions to stay on a reference speed Vref with power.

 

[FastEddieB]

Is it useful to think about how an autopilot does it? Mine used pitch for glidepath control and I used power adjustments to maintain airspeed. I’ve never flown with autothrottles, but my assumption is they adjust power to maintain airspeed. So it seems like when aeronautical engineers had to design autopilots they went with pitch for glideslope and power for airspeed as the most efficient way.

 

[Martin Pauly]

Is it useful to think about how an autopilot does it? Mine used pitch for glidepath control

Well, of course it uses pitch - unless you have autothrottle, what else can the autopilot do?
In light GA aircraft, an autopilot can control a vertical flight path (i.e. follow the GS, or maintain altitude, or maintain a vertical speed) or maintain a desired airspeed (i.e. FLC or IAS mode, typically used in the climb), but it cannot do both at the same time.

- Martin

 

[dmspilot]

No autothrottles on the jet I fly and watching the autopilot fly a glideslope in bumpy air is super annoying. Easier just to hand fly so that you can make pitch and power adjustments simultaneously.

 

[nauga]

The downside I see with only preaching "pitch for airspeed, power for altitude" is that you'll find pilots adding power when they are half a dot below GS while 15 knots too fast. In that case, just pitch up a bit instead, and convert kinetic energy into potential energy. Same goes for being high and slow - just push the yoke forward, and kill two birds with one stone. All too often, I see pilots in training focus on just one of those at a time.

I have also seen pilots pull up when low and slow or pitch down when high and fast. The problem is not with teaching a pilot either (or both) technique(s), it's simplifying stuff so much that, as you implied, glideslope and speed management are treated as two independent processes.

Nauga,
MIMO