3 degree glide slope tricks?

[Lance Link]

I've always been told that identifying a 3 degree glide slope, absent IFR instrument help or lights, just has to be done based on experience.  Any other tricks?

In particular, if I am on final, full flaps at 70 knots in a J model for example, and the HSI shows 3 degrees, am I on a 3 degree glide slope?    I gather that would mean the plane is flying straight down the glide slope line and is not pitched up any to maintain the 70 knot approach speed.  If this is true, seems like it would be a fairly good cross-reference.  

Any thoughts or tips?  Thanks!  

[Andy95W]

FAF altitude.  If you cross at the altitude on the approach plate, you're on the correct glideslope.

[Lance Link]

Excellent suggestion Andy, never thought of that.  Thanks.  

[FloridaMan]

Bear in mind if you're flying a coupled approach and are way high on the ILS, you can get a false glideslope and your autopilot may attempt to stall you. 

[Immelman]

3 hours ago, Lance Link said:

I've always been told that identifying a 3 degree glide slope, absent IFR instrument help or lights, just has to be done based on experience.  Any other tricks?

In particular, if I am on final, full flaps at 70 knots in a J model for example, and the HSI shows 3 degrees, am I on a 3 degree glide slope?    I gather that would mean the plane is flying straight down the glide slope line and is not pitched up any to maintain the 70 knot approach speed.  If this is true, seems like it would be a fairly good cross-reference.  

Any thoughts or tips?  Thanks!  

1) the FAF altitude check as mentioned is an excellent idea in that it also validates the glideslope is working correctly and that you're on it. There have been several cases where airliners have nearly crashed due to a false glideslope indication that could have been caught this way. A recent incident involved a field being changed around (landing one direction to another). A taxiing large aircraft was parked near the glideslope transmitter which screwed it up. There was no cockpit indication of failure, only the glideslope beam in the wrong place in the sky....

2) Vertical speed = groundspeed * 5. That works for 3 degrees. Another way to calculate this on the fly is to multiply by 10 and divide by 2. These are only mental math tricks but it is an important tool. Glance at your GPS groundspeed once established, and quickly calculate your target vertical speed. This will keep you tracking the glidepath, and adjustments are made from there. You can work this out in your spare time with a paper, scientific calculator, and some basic trig.

I really don't pay much attention to the pitch attitude or specific power setting. Are you flying fast and going to conduct a decelerated approach? Flying slow? Flaps, no flap? Those will change the required pitch. Headwind? No wind? Tailwind... wind shift... all those will change the required power.

 

3) I'm confused when you said "The HSI shows 3 degrees". HSIs can display lateral course deviation in degrees (i.e. from a VOR) but LOC deviation is specific to beam width. Glideslopes vary in their angle. Most are 3 degrees but a few are shallow and a few are steeper. The glideslope angle is on your approach chart. Either way the glideslope receiver in the aircraft and HSI displaying deviation have no idea what the glidesope angle is, only deviation from the center.

Edited June 20, 2020 by Immelman

[EricJ]

23 minutes ago, FloridaMan said:

Bear in mind if you're flying a coupled approach and are way high on the ILS, you can get a false glideslope and your autopilot may attempt to stall you. 

That's another reason to hit the FAF at the proper altitude:  it guarantees you'll intercept the ILS from below and not lock on the alias/reflection/sidelobe/false glideslope.
 

[MB65E]

300ft agl per nm from the end of the runway. 
3mi final= 900ft AGL.
30Nm=10k ft. 
Common big numbers to keep you on slope. 
Its not as easy as it sounds. In the past with most light airplanes I could always “fix” the approach. The Mooney requires a stable approach but one can still fix the approach in a Mooney. Other larger faster aircraft, each “gate” needs to be made on speed and ALT because there is a point where you just can’t fix it anymore. The process still requires some thought for me. Most systems won’t allow you to intercept above the path. Gotta have that GS intercept altitude in your head prior to anything really. 
-Matt

[Matt Ward]

Also, and I suppose to each his own, but I am established on glide well before full flaps. My config is set with 15 degrees at the FAF (if not circling) and no more until visual. 

[carusoam]

It doesn’t look like Lance has given us a hint to IFR, or how far out he is talking about...

But... visually, final approach requires some lights at the end of the runway.... lights provide a visual glide slope...

Sometimes lights aren’t available...

If we are traveling at 70kias for final approach,  some people select a 400fpm descent...     Being on target, speed and altitude, at key points in the traffic pattern... allows for the full flap approach to the landing zone...

Coming in too high or too fast... starts to get less controlled.... stronger adjustments are required...

As the vertical speed gets higher, the approach de-stabilizes...
 

Quick...

• HSIs don’t show a 3° approach, unless you are following a glide slope...
• AIs won’t show a 3° approach, because you can maintain attitude and descend wildly...

So... Lance, I think you hit on something... it takes experience to judge a 3° glide path when instruments and light signals are not available...

You can always look for a Vasi or Papi in the airport info during the flight planning exercise...

Be ready to adjust power to stay on glide slope...

PP thoughts only, not a CFI ...

Best regards,

-a-

 

[M20S Driver]

2 hours ago, Immelman said:

2) Vertical speed = groundspeed * 5. That works for 3 degrees. Another way to calculate this on the fly is to multiply by 10 and divide by 2. These are only mental math tricks but it is an important tool. Glance at your GPS groundspeed once established, and quickly calculate your target vertical speed. This will keep you tracking the glidepath, and adjustments are made from there. You can work this out in your spare time with a paper, scientific calculator, and some basic trig.

 

1 hour ago, MB65E said:

300ft agl per nm from the end of the runway. 
3mi final= 900ft AGL.
30Nm=10k ft. 
Common big numbers to keep you on slope. 
-Matt

+1 to both.   I just use 1000 feet in three mile final and set them up as the starting point and double check the speed at 500 feet again.  

[Lance Link]

I suspect I was too sparse in the information I provided.  Here I am talking about VFR approaches with no PAPI lights, ILS or anything like that.  Just my eyes, the airplane, and the threshold.  Now, I fix the aim point in the windscreen where I think it looks like 3 degrees, from limited experience.  Also, it occurs to me I meant attitude indicator, not HSI (duh).  So carusoam, you answered that question, the AI won't tell me if I am flying a 3 degree slope, because the pitch is too variable.  I suspected as much.  Otherwise we would have all been taught that!  And, I note that in Don Kaye's landing video, his VSI does show about 400 fpm most of the time.  Although he's in a Bravo.

300ft agl per nm is a good rule of thumb tip, that does translate to about a 1:20 slope.

And backing into vertical speed based on ground speed * 5 is also a good tip.

I will try those things to help with landing at some of the out of the way strips I am exploring now that the airport restaurants are still closed.  I am trying to get this dialed in to smooth out landings and so I don't have to keep wondering if I am high or low.  

Thanks everyone for the suggestions!

 

 

  

[carusoam]

Feel free to use your new knowledge when you have all the other things available to you...

This will allow you to test your new knowledge against the tried and true visual approach standards...

Typically we have 1k’ to lose in a short period of time...

If we accidentally delay our descent...  we will be forcing something at the other end....

If you momentarily achieve 1kfpm... in the descent... expect that you can do better the next time around... 

If you have done IR training... there is an empty feeling associated when you can’t clearly identify the FAF...  you want to get started down... but you can’t start until you know you are there.... Time starts clicking away... 

GPS makes identifying the FAF so easy...
 

Great discussion points Lance!
 

Best regards,

-a-

[Immelman]

1 hour ago, Lance Link said:

I suspect I was too sparse in the information I provided.  Here I am talking about VFR approaches with no PAPI lights, ILS or anything like that.

 

 

  

Sorry I thought you were asking about an ILS approach.

For visual approaches in a single engine piston airplane - far away from the runway you could do distance * 3 = 1000's of feet AGL to get you in the neighborhood. This is descent planning. Closer in to the runway, i.e. after you leave traffic pattern altitude, throw all that stuff out: your primary reference outside is the sight picture of runway to horizon, and primary inside reference the airspeed indicator. Keep the ball in your scan to ensure coordinated turns. There really isn't anything else that's needed. Keep it simple. Don't look at your attitude indicator. Don't look at the GPS, hell don't look at the engine gauges. Airspeed, outside, airspeed, outside.

Admittedly my technique is based on a bunch of time and landings in the Mooney. To start off with, I can understand doing some exercises to get your initial power setting, airspeed, and configuration sorted out, a starting point of sorts to then go practice landings. Get a good starting point figured out and then practice and refine. IF you desire to fly a 3 degree path (and that is not the only choice), practice on a runway with a VASI or PAPI. That will help train your eyes for a 3 degree sight picture.... but then beware, that sight picture will be significantly altered with a sloping runway.

Practice... more practice... power on approaches, power off, wind, no wind (oh, how the wind makes such a difference). It will come.

Edited June 21, 2020 by Immelman

[jaylw314]

8 hours ago, Lance Link said:

I've always been told that identifying a 3 degree glide slope, absent IFR instrument help or lights, just has to be done based on experience.  Any other tricks?

In particular, if I am on final, full flaps at 70 knots in a J model for example, and the HSI shows 3 degrees, am I on a 3 degree glide slope?    I gather that would mean the plane is flying straight down the glide slope line and is not pitched up any to maintain the 70 knot approach speed.  If this is true, seems like it would be a fairly good cross-reference.  

Any thoughts or tips?  Thanks!  

Your descent rate should be your groundspeed x 5 (like @FloridaMan  said).  So find an airport with a 3 degree PAPI or other visual glideslope indicator.  Get on the glideslope and stabilize your descent rate to 300-350 fpm at 70 knots with full flaps. 

Now look at where the runway touchdown point is between the airplane's nose and the horizon.  That picture should always look the same if you're at 70 knots and descending at 300-350 fpm.  In my plane, it's pretty much halfway between the nose and the horizon.

Obviously, if you're coming in faster or you have half or no flaps, the picture changes, but soon you will figuring out your glide angle by the shape of the runway (better but not perfect), and finally you'll figure out the distance between the horizon and the runway (which is actually the glideslope angle)

[Andy95W]

3 hours ago, Lance Link said:

I suspect I was too sparse in the information I provided.  Here I am talking about VFR approaches with no PAPI lights, ILS or anything like that.  Just my eyes, the airplane, and the threshold.    

Sorry, I thought you were talking about an ILS also.

As to being VFR with no visual or electronic help, it is true that practice and repetition help.  With that said, I've been practicing for 30 years and thousands of hours and I still screw it up (albeit not quite as often or as badly as I used to).

[Lance Link]

I apologize for not making the question clearer.  Lots of good tips here and material for practice.  So much good information is available on this forum, can’t put a price on it.

[Hank]

1 hour ago, Lance Link said:

I apologize for not making the question clearer.  Lots of good tips here and material for practice.  So much good information is available on this forum, can’t put a price on it.

I like to fly the VFR pattern at 1000' and 1/2 or a little further out (but well under a mile!). Gear down to start descent abeam my intended landing spot; turn base when the runway is 45° behind me (~700'agl), then turn final to line up with the runway (~500'agl).

Then it's just power, elevator and trim to reach the runway. But I learned and based my Mooney for 7 years at an obstructed 3000' field, so it's just what I've always had to do.

Spent 2 years with 5000' and dual Instrument approaches, which spoiled me, then back to 3200' with.no lights or approaches; now I'm at 5000' and sometimes get to choose uphill or downhill . . . . 

Practice, practice, practice! None of us have enough lately. My landings have become merely functional. 

[EricJ]

2 hours ago, Lance Link said:

I apologize for not making the question clearer.  Lots of good tips here and material for practice.  So much good information is available on this forum, can’t put a price on it.

I think one reason for the confusion is that maintaining a 3-degree glide path isn't that critical for VFR approaches and some might consider it too shallow.   Many PAPI (or even instrument) approaches are set at 4 degrees or even up to 4.5.    Since winds, terrain, traffic, noise abatement and other factors come in to play frequently in VFR approaches the angle is probably not that high on the priority list.   These days people do try to hit the PAPI (or VASI or whatever) lights, but those are aids rather than absolutes.   So from that perspective I think one of the better techniques is just judging whether the desired target point is moving up or down in your windscreen.   If it is moving up, you're unlikely to make it in your current configuration, and if it is moving down, you're likely to overshoot.    This works for pretty much any stable approach, regardless of angle.

[midlifeflyer]

17 hours ago, Lance Link said:

HSI shows 3 degrees, am I on a 3 degree glide slope?

Yes, but no No NO!

You are asking about "absent IFR" and I think that means a visual approach. That's a visual maneuver and should be done with outside references. You should be able to maintain the proper path - horizontally and vertically - and at the proper airspeed, with the instrument panel completely covered. 

A couple of tips to develop that (although  I pull it on pilots all the time, and will not even solo a student until they can, chances are you can already do it - no one so far has had any difficulty). 

• use the VASI or PAPI, but as a training aid to learn what the glide path looks like without them. In theory, you  can do that with a needle, but the idea is to get you looking outside. 
• groundspeed X 5 is a target for a 3 degree path. Yes, using IAS,  there will be variation for wind and density altitude, but we are using it as a training aid, not as an instrument target.

[V1VRV2]

If not on a ILS approach then it’s called a glide path. On an ILS approach it’s called a glide slope. A 3 degree glide path can be referenced by an approx  750’ descent rate (see correction below) on the VSI absent of any PAPI, VASI. It’s just for reference however. A visual approach you should have your eyes outside. Yes... the correct 3 degree approach picture comes only with experience. Pitch for the correct picture out the window and power for airspeed. Don’t change that sight pictur when on the correct glide path.

 

PS.... I was thinking big plane which I usually fly..... the correct descent rate for your speed can be referenced by Jeppessen approach plate for any ILS approach. Most approaches flown at 70kts ground speed gives you a 350’ descent rate. If there is an ILS approach to the runway you want to use reference it for the proper descent rate even for a visual approach. On the approach below proper descent rate for a 3.1 degree glide slope/glide path is 384 ft min.

 

PSS If your not instrument rated it doesn't hurt to dial in the ILS and reference it on a visual approach if you plane is so equipped. Airline world... all our visual approaches are backed up by a published approach to the runway if available. 

Edited June 22, 2020 by V1VRV2

[Ibra]

3deg is 5% gradient, 1:20 glide, that has to be with lot of power as your 90kts M20J best glide engine off will be 1:10 (or maybe 90kts tailwind & no power ), for 1000ft all you will be about 3NM, that is really shallow the runway will show on the horizon (or just 3deg bellow )

Do you need that for VFR? absolutely not, in theory, you can achieve any flight path up to 1:3-1:6 (about 1/2 nm to 1nm per 1000ft as @Hank mentioned) without getting into uncomfortable very slow or very fast speeds you just need lot of practice to come that steep (30% gradient, 20deg, 1:3 path at 70kts, but remember descent rate will be in 2000fpm), just need practice to burn height & maintain speed

All these can be done with ASI only and everything covered, 1:20 is really shallow for VFR, 1:10 should be about the norm for VFR, 1:4 needs lot of practice and will look interesting, anything  bellow 1:4 is STOL competitions or helicopter glides (or auto-rotation or whatever they like to call it to feel special)
 

Edited June 21, 2020 by Ibra

[Flash]

4 hours ago, V1VRV2 said:

If there is an ILS approach to the runway you want to use reference it for the proper descent rate even for a visual approach. On the approach below proper descent rate for a 3.1 degree glide slope/glide path is 384 ft min.

 

PSS If your not instrument rated it doesn't hurt to dial in the ILS and reference it on a visual approach if you plane is so equipped. Airline world... all our visual approaches are backed up by a published approach to the runway if available. 

I agree with V1VRV2 that it's good practice to dial in the ILS even if you're not flying the approach. OTOH, you should be careful if you're flying this approach not to get distracted by the glideslope; the approach plate warns you that "VGSI and ILS glidepath are not coincident." If you're flying VFR or flying a visual approach, you probably should be following the visual glideslope indicator rather than your plane's glideslope indicator, especially because, as V1VRV2 mentioned, your eyes should be focused on what's outside the aircraft, rather than what's showing on your panel.

[Flash]

Another interesting thing about this approach is that coupled approaches are not allowed. My understanding is that this means there's risk of temporarily incorrect guidance that, if the autopilot were flying the approach, might lead to a dangerous "chasing the needle" reaction, whereas a human would not overreact or overcorrect in the brief time that the signal is incorrect. (Didn't mean to hijack the thread away from 3 degree tricks, but thought this was noteworthy.)

[adverseyaw]

Reading back to the original question, I think using the FAF is the best way to dial in what a 3-degree sight picture looks like.

If your goal is to learn to identify and fly a 3-degree visual approach, those should be treated as separate skills.

1. Identify a descent point so your approach is 3 degrees (this is the FAF technique @Andy95W described)
2. No matter which descent point you pick, fly a constant-descent approach to the runway

FAF and an ILS are the only way I can think of to achieve step 1 with precision. You can use your FPM to see if you're way out of bounds but I don't think it will help you dial that sight picture in.

Step 2, the actual descent, is best done visually. You don't want to use a descent angle for step 2 as any inaccuracy in step 1 or in your groundspeed estimate will put you increasingly off your glideslope, which defeats your goal.

 

In addition to the rules of thumb for calculating FPM, Advisory Circular 120-108 includes a table of constant descents (Appendix I, Figure 3) that will give you precise numbers . In reality, wind gradients throw all of this out the window, and those descents are usually only used to ballpark an approach until the pilot can get back to reliable indicators (DME, marker beacons, and altitude).

https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/877339

 

[Lance Link]

This is all very interesting and informative to me.  I had never thought about using the FAF, for example, to better teach myself to see the glide path.  (I do that with PAPI lights of course but kind of consider it cheating).  It a great idea and I'm going to try it.  I started IFR training right before the covid hit, but I did learn enough to enable me to used approach plates to work on this I think.  That in combination with the other tips will mean a lot of methods to practice, and combine to dial in a system that gives me consistency.

Here is something else interesting:  I got curious about this, so I did the trigonometry, and then went to Google Earth and Fore Flight 3D and looked up some out of the way airports I've landed at, and where I tended to turn base based on eyeballing.  I realize I been flying VFR approaches too high forever!  I was initially taught to turn base in a 172 at 400 ft agl 1/2 mile from the runway.  What the.....

For anyone interested, I found the tangent for 3 degrees is .052, and for 4 degrees it's .069, unless I'm bad at math.  So to figure out how far from the runway I should be for a given glide path, I divided my relative altitude at the point where I turn base by one of those numbers.  Based on landmarks, I seem to always be turning too early and/or too high.  And I do realize 3 degrees is really shallow for a light GA plane.  I might find higher looks and feels better.  But first, I need to figure out how to know where the heck I am.