Menu Sign In Contact FAQ
Banner
Welcome to our forums

Flying the "glideslope" on a nonprecision approach (constant ground speed)

Example:

To arrive at the MAP correctly, you have to fly the correct vertical profile. That means flying an accurate ground speed – either one of the figures listed in the table, or some interpolated value, and an accurate VS.

In the above, for a 100kt GS, the VS needs to be -630 fpm. If you fly say -500fpm you will end up too high at the MAP and deny yourself the opportunity to be visual. If you fly say -700fpm you will be too low at the MAP and will reach the MDA before the MAP, with the same result.

The VS is taken care of with a modern autopilot, which will do the lateral nav at the same time.

So you just need to arrive at the FAF correctly configured (usually, gear dropped and Flap 1 at the FAF) and doing the chosen GS, and control the throttle on the way down.

It’s a bit tricky because you have to hold the GS, but at the same time you have to watch the IAS to make sure that is within safe limits.

With a strong headwind your IAS could easily drop too low, even with a GS of say 100kt, especially in the initial stages where you are still high up.

It’s an interesting scenario, not trivial with a nice autopilot, and a real bastard to fly well by hand.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

It’s an interesting scenario, not trivial with a nice autopilot

Why? My airplane would fly that completely on its own with a virtual glideslope. If the Jepp plate has a straight line from the FAF, the Garmin navigators have an advisory glideslope which to the autopilot is the same as a “real” one. I’d just load the approach and let the AP fly it.

One doesn’t fly a ground speed at all. Fly the approach at a fixed IAS and take your target VS from your ground speed at that IAS. You should then be using the height/distance table (I assume there is one that you have cropped out?) and adjust your VS based on whether you are high or low at each check height. It is a fairly high workload either with or without automatics, and one of the situations I really prefer a paper plate clipped to the yoke to fly.

If you have a strong headwind, you just accept whatever groundspeed you get at that IAS. I would imagine if you fly at 100kias and 600fpm you wouldn’t go far off and can just tweak the VS to fly a profile that will at worst leave you 3w/1r or 3r/1w when visual to adjust onto. You will probably pop out 3W/1R anyway if the runway has 3° PAPIs and a 3.56° path on the approach.

London area

If there’s no virtual glide slope and it’s a LNAV only then I’ll make a DCT to the Runway and with the help of VNAV and your right hand on the VS knob of the autopilot adjust the VS according to the calculation of the VNAV function of the 430.

Or make a DCT to the Runway and set the MDA and fly it with the Flight Director of the DFC90, which is a lot of fun.

If there’s no published LNAV/VNAV or LPV then of course you don’t have a virtual glide slope (not with the GNS430W), The Garmin 1000/Perspective OTOH can create a GS to ANY point, like the threshold of an uncontrolled VFR airport.

@ achim: Which airports that have no LNAV/VNAV or LPV have an “advisory GS” in the database of the GS430W you can use with the DFC90?

Last Edited by Flyer59 at 03 Nov 12:51

Where I fly (Australia) charts normally have a table with distances/heights. That’s not the case on these charts?

There is a rough formula for 3 degree glideslopes: multiply GS x 5 = RoD required. I.e. 110kts ground equals 550’/min to follow a 3 degree glideslope.

Last Edited by Archie at 05 Nov 19:28

Archie wrote:

110kts ground equals 550’/min to follow a 3 degree glideslope.

Yes, because 3° is roughly 5%

LFPT, LFPN

This is relatively basic stuff that any IR pilot should be able to follow. The correct technique is to estimate/calculate groundspeed and then set the recommended RoD at top-of-drop, cross-check ranges against target altitude and adjusting RoD as necessary. Don’t try to adjust IAS – keep this constant and just manage your RoD. A NPA like this actually lends itself more to throttle for altitude and attitude for speed rather than point-and-power.

I disagree that it is a “real bastard” to fly well by hand, especially considering you minima is normally going to be in the 350ft territory (OK, in some scenarios it can be as low as 250ft).

Last Edited by Dave_Phillips at 05 Nov 19:42
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

It’s interesting to read this.

I was never taught how to fly a NP approach, in terms of speed control.

So I developed my own “fly the ground speed for which you have set the VS on the autopilot” method.

Yes, Shoreham EGKA does have a couple of points at which you check your profile

but we are talking about how to best fly the continuous descent line.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I was never taught how to fly a NP approach, in terms of speed control.

It’s a big portion of the EASA IR education and I have to demonstrate my ability to do so every year as part of the checkride. It requires one precision and one non precision approach and as part of the briefing I have to tell the examiner my expected sink rate (taking wind into account) and have to demonstrate where I am at each altitude step in the table and take corrective measures.

Peter wrote:

So I developed my own “fly the ground speed for which you have set the VS on the autopilot” method.

Luckily my AP, thanks to the GNS430W with software 5.x, can fly them all for me so I only need the skills once a year for my checkride

Peter wrote:

Yes, Shoreham EGKA does have a couple of points at which you check your profile

That’s something else, you are not allowed to undercut those points (see the gray blocks). Normally there is no such restriction but in order to demonstrate a continuous descent, you use the table just above (DIST to RW20 in the EGKA case) which is present on every chart. If you are below the check altitudes that you highlighted, the examiner would fail you. If you are below the altitudes in the table above, the examiner would expect you to recognize the deviation and correct your glide.

In traditional IR you hand fly everything. I was taught to hold the VS but of course that will give you the “glideslope” only if your GS is constant too. And you don’t know the GS unless you have a GPS (or DME).

On the ILS you also maintain a VS falue, and adjust it periodically to hold the glideslope.

Sure one must not bust those two step down fixes (SDFs).

Most of the time one is not flying anywhere near minima so ending up well above the “glideslope” is fine. You still “look good” on the checkride. It is when flying a NPA to the minima that you need to do it just right.

Administrator
Shoreham EGKA, United Kingdom
54 Posts
Sign in to add your message

Back to Top