VFR Pattern Glide Slope, as compared to IFR 3 degree GS

9 Answers

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   Gary Moore on Jan 17, 2012

   Well – I’d be careful telling my CFI he was crazy 🙂 
   I’ll trust your math here – but I think you may be over analyzing it just a bit. The odds of you being EXACTLY 1/2 mile or EXACLTY 3000′, or holding EXACTLY 500’/min is pretty slim. I’m guessing your CFI is giving you ‘approximate’ guidance in the early days of you mastering the landing.  You will, over time, develop a feel for what is the right altitude, when to turn, are you too high/low etc.  But it takes lots of practice in the plane – not on the calculator.  Give your CFI a chance to guide you through this learning experience….

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   Jack Dander on Jan 17, 2012

   Thanks Gary.  I trust my CFI with my life, of course, proving it when I go up.  It turns out he was RIGHT about stuff above and the 3 degree folks I read in the WWW were wrong, at least IMHO.  I will continue to learn from him as I complete my BFR.

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   Brian on Jan 17, 2012

   “Total of 10,500′ at 1.4 Vso and then 1.3 Vso.  So, 1000′ AGL in 10,500′ is 5.4 degrees.”

   You’re complicating the heck out of it. Fly at best glide speed, change speeds when you’re over the runway to land.

   If you’d like the math, at best glide speed the 172 published glide ratio is 9:1 — so Inverse tan (1/9) = 6.34 degrees. You’re more likely, given the plane isn’t freshly polished out of the factory, to get an 8:1 glide ratio which comes to 7.12 degrees.

   Even the above math, however, isn’t much use for you in the aircraft. You’re better off thinking about what visual cues to use to help predict and succefully make your intended point of landing. Secondly, what procedure will be best. And the key to a procedure is you must do it the same way every single time.

   Here’s my procedure (not the only one in existence): Simulated engine failure. When abeam the landing point, 10 degrees flaps and trim hands off best glide speed. Turning base I put 20 degrees. Full flaps is reserved for short final or as necesary to save a botched approach.

   Note: For practices like this, if I fail to succeed with a stable approach only using flaps, I don’t slip to make it work. This isn’t to say I never practice slips, just not now. Sure, in a real failure I would slip as needed. But my goal here is to make a perfect approach, and if it isn’t perfect I throw it away and try again. Once I have it perfect I practice that perfect approach so the visual cues can be engraved into my head.

   Visual cues: What you’re looking for is a point on the windscreen that remains stationary. I’m going to borrow a snippet from the book stick and rudder to try and better explain. The author mentions astronomers to help us understand how we can judge our glide. When an astronomer tells another astronomer how to find a star in the sky they say something like, “look to heading 275 and shift your eyes upward from the horizon 35 degrees.” Picturing the sky as a big bowl, or semi circle, an astronomer can give an darn accurate explanation of where to find something in the sky using a heading and degree up from the horizon. To help visualize you can draw a half circle on a blank piece of paper and then draw a line that makes a 35 degree angle with the center of the circle.

   The author goes on to tell us that we can use the same idea in an airplane. In a no wind condition, at best glide speed, the airplane will have a constant gliding angle. So if we were to go up on a windless day (or fly a heading on a windy day that puts us in a crosswind, so there is little or no headwind/tailwind component) we will find a point on the windscreen that remains fixed through our glide. Eventually, if we kept gliding, we would smack into that point. That point will be a certain number of degrees below the horizon, just like the astronomer’s stars are a certain number of degrees above. Memorize how much you shift your eyes from the horizon down to this aim point and bingo, that point will always be the same whenever you don’t have a headwind/tailwind. In other words, anytime you fly cross wind.

   If you were able to follow the above, and yes it is quite tough to follow at first. Took me a few reads and many flights before I had a clue. So don’t get too down, just reread it and ponder the idea. When you feel you understand it you can expand the idea. The degree’s below the horizon works regardless of which window you look out of. Ahead of you will of course give you the point you hit straight ahead, but look to your left. If you were to shift your gaze down from the horizon the same number of degrees out of the left window, just as you do off your nose, you will see what you will hit if you turned and pointed the nose that way. What a wonderful way to determine when it’s time to turn final from base….

   The ideas presented, especially with regard to visual cues, are no doubt complicated. The author of stick and rudder tells us that this concept is the capstone of flying an aircraft by visual cues. It takes many hours to finally graps. So don’t beat yourself up, keep working on it, ask questions if you’d like and I’ll do my best to answer, and you will get it.

   That said, these cues and a consistent procedure is where your focus should be, not on calculating unusable numbers and over complicating simple procedure. Because, when the day comes that your engine fails, the simple stuff is what you’ll remember and do correctly. The rest will be white noise at best, and cause you to second guess what you know at worst. 

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   Ross on Jan 17, 2012

   I’m not a CFI but a private pilot.  Your CFI is just giving you these numbers as a way for you to visualize where you are in relation to the process of landing.  Good to understand on the ground with a pen and paper but Gary hit the nail right on the head.  At some point you’ll be at a point where you just know by glancing at the runway if you are too high or too low.  It’s all about speeds and the feel of it!  85/75/65 centerline!  

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   John D. Collins on Jan 17, 2012

   My guess is that at this point in your training; your flight instructor is trying to give you some visual cues that along with some procedural points will help you with landing while in the traffic pattern. Eventually, as you become experienced, you will be able to adjust to meet the conditions.  At this point, you are over thinking it, as almost every pattern will be different in some degree. The winds will be different, the distance from the runway will be different on downwind, and when there is traffic, you have to make your turns to provide spacing.  When I was learning to fly, my instructor gave me power settings, flap settings, and target airspeeds.  It was a long time ago, but if I recall, he wanted downwind power at 2000 RPM, mid field first notch of flaps, abeam the threshold reduce the power to 1500, turn base when the runway was 45 degrees behind my wing, add another notch of flaps, turn final, and go to full flaps. Then reduce my power to keep the numbers at the same spot on the windshield.  I don’t remember the speeds, but I think it was 60 MPH on final in a Cessna 150.

    

   Now a days, with a VASI or PAPI, you have a visual glidepath for guidance, although the angle is typically around 3 degrees.  If you are in a C172, this will be too shallow for a power off glide and you will need to carry some power if you attempt to follow it.  I would not expect you to be bound by the 3 degree slope, except if you see yourself low, you will want to add power and maintain altitude at least until it indicates you are not low. 

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   Brian on Jan 17, 2012

   “My CFI wants me to be 1/2 mile from runway, 1000 ft AGL,  on downwind and reduce power opposite the 1000′ mark on runway.”

   My apologies, my entire post was in regard to power off gliding as I misread your post. The visual cues are still applicable as long as you use the same power setting each time. Winds will cause a slight variance, but negligable when +/-10 knots. It would seem John and Gary addressed the rest.

   I’d have one add to my post from eariler (I know it was already too long, right? -__-) The number of degrees below the horizon you shift your eyes is relative to the glide angle. That is, the steeper your glide angle, the farther your eyes will shift below the horizon to find the aim point. That said, anything that increases your glide angle — flaps, slip, headwind — will cause a steeper glide angle and your gaze will have to shift a number of degrees farther below the horizon as such. The opposite for a tailwind — a more shallow glide and eyes shifting fewer degrees below the horizon.

   Good luck and apologies for any confusion I may have caused.

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   David Eberhardt on Jan 21, 2012

   If you’re practicing engine out approaches, you’re glide path angle will be steeper than the vasi (or papi) glide angle of 3 degrees. How many instructors and pilots practice engine out glides according to the manual (no flap)? That would require gliding at best glide speed with flaps retracted to get the maximum glide distance. With that sort of glide, you still would not be able to maintain a vasi glide path 
   If you fly a 1/2 nm spacing on downwind and plan to roll out on final at 1/2 mile (@ 400 feet or so) – you’re gonna need to start lowering the flaps in increments, unless you plan on landing on a long runway, well past the threshold. That sort of set up means you’ll need to lose 1000 feet in 1.5 miles (you need to lose about 666 feet per mile – so to be at 400 feet at .5, you need to glide a little shallower during downwind and base leg descents. You’re instructor is correct.)
   I think you have the right idea, that initially you’re descent on downwind and base leg would be shallower than what it will be on final – all becasue you keep adding flaps as you go. As you lower flaps, you’ll have to lower the nose a bit to keep your airpseed up. Final approach would see a descent rate of 800-1000 feet per minute. 
   I like to practice eng out patterns using about 1/2 mile spacing. 1st notch of flaps when I start my descent on downwind … turn base and then add 2nd notch of flaps at the middle of base leg. I also check to see that I am about 600 agl at the half way around point. 
   Because every pattern is different, the use of flaps is the key. If I have strong headwinds present for final approach, I delay my flaps extensions a little.If I feel that I am a little tighter to the runway or winds are calm, I may need to get the flaps down a little earlier. It all depends. This is where practice will be the teacher.
   Select full flaps when there is little doubt that you can make the runway. Sometimes I don’t get to full flaps. That only means my touch down speed will be about 5 knots faster than optimum. Not catastophic.
   I have a video of one of my practice sessions on you tube if you’d like to see it. It’s hard to see but the PAPI lights are located about 500 feet past the threshold BUT I am aiming for the runway numbers which are about 250 feet from the threshold. I would be above the PAPI glide path early on the approach and ultimately below that glide path on short final.
   eng out (simulated) video – http://youtu.be/bcuqYNkeoGE

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   Jack Dander on Jan 23, 2012

   David, and others…..  your answers are spot-on. Thank you.  I do overanalyze stuff I suppose, but David’s answer, especially, is what I would expect to see in a Flight Training magazine (see 2/12 issue) (that incessant 3% g.s. stuff!).  If flight training is initially in small trainers and small trainers need to come in at 600 to 800 fpm or more. then why not say that to students in those type of journals and magazines and books and www?  Thank goodness for CFI’s.  But I wonder about some student reading that other stuff and in a solo moment dragging his dirty config’d small a.c. 1 mile at 300 feet on final.  Ugh.  

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   David Eberhardt on Jan 23, 2012

   I just looked at Ian J. Twombly’s article in Flight Training magazine called “The Last Six Inches”. In it he says to arrive on a 3/4 to 1 mile final at 400 feet. That would make for a 4+ degree glide angle .

   In your response, I could not tell whether or not you understood he was describing a 4 degree glidepath, but he is.

   I don’t have a big problem with flying 3 degree glide paths, after all – most VASI and PAPI systems are 3 degrees and it helps some people with “seeing” their glide angle better. Following the VASI or PAPI at NIGHT is probably a pretty good idea too. So a little practice with the VASI and 3 degree is not all that bad.

   I actually like the fact that you take the time to crunch some numbers and try and thoroughly understand as much as possible as it relates to aircraft performance and such. I just wanted to provide a simpler way – i.e. 400 feet at 1 mile = 4 degree glide path … you can look at the back page of an instrument approach book and find descent RATES based upon glide angle and ground speed.

    

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