dasleben
That's just, like, your opinion, man
We have one at the airport I'm based at. There's a fairly good amount of terrain, so it's actually set at a 3.5 degree descent. Very strange, that.
Descent Below VASI on Approach
- Thread starter Zero1Niner
- Start date
We have one at the airport I'm based at. There's a fairly good amount of terrain, so it's actually set at a 3.5 degree descent. Very strange, that.
R
Roger, Roger
Guest
Meh. One day I'll see the light and be as conservative as you. For now, I see nothing wrong with hitting the threshold, especially in a piston single like a 172.
seagull
Well-Known Member
Until you hit that last moment downdraft or have your engine fail on short final when you needed a little power to make that threshold....
I'm far from conservative...just experienced enough to know that I don't need to save a minute by ducking under a glide slope.
Scared myself one too many times doing that as a CFI. Luckily, once was enough. The aiming points are there for a reason...aim for them.
-mini
deadstick
Well-Known Member
I was just looking in the ol' OPSPECs, C053 I believe, and remembered this thread. In the part that copies 91.175, it lists "Visual glidepath indicator (such as VASI, PAPI)" instead of "The visual approach slope indicator." If anything, if it's good enough for 135 it should good enough for the fun-lovin' 91 crowd, right?
I don't think that's necessarily true. How many part 91 pilots get to pick an airport for an alternate that's forecasting 400-1 weather at the ETA? You can under 135...assuming the airport meets the requirements.
-mini
I don't get it, why would they? Ground roll is a function of kinetic energy which is directly related to airspeed. How much ones ground roll is has to do with their airspeed, not the descent angle chosen.
blackhawk said:
Just so others know, this was blackhawk's interpretation of regulation 91.129 (e)(3): Each pilot operating an airplane approaching to land on a runway served by a visual approach slope indicator must maintain an altitude at or above the glide path until a lower altitude is necessary for a safe landing.
I agree with that interpretation. Once below the DH/DA of a typical precision system or any airport served by a visual guidance system that should be adequate to be safe to descend below, hopefully the FAA would agree.
Where to land? The numbers or the thousand foot markers, it seems this debate trickled over to this thread. I will speak only of GA aircraft in a quick analysis I would like to throw out here for others to examine.
A typical glide path is built on a 3 degree slope, this would be a glide ratio of approximately 19 and 1. With this glide slope you cross the 200 foot AGL point at a distance of 3816 feet. If one was aiming for the thousand foot markers they would be 2816 feet from the numbers.
Let's just throw in some actual glide ratios now, the C172 glides at 9 to 1 according to the POH, from readings I have seen people experiment getting numbers as high as 12 to 1. Each of these assume no flaps, and obviously aren't calculating glide ratio with full flaps power idle. I would guess with full flaps power idle it is closer to 6 to 1. So I did some calculations based on that.
Let us recall just a paragraph ago that if we aimed for the numbers we must be at least 200 feet or higher at 2816 feet from the runway to remain on or above glide slope. Here is a list of distances at an altitude of 200 feet for the glide ratios mentioned above:
12 to 1 results in 6.4 degrees and a distance of 2382 feet
9 to 1 results in 4.8 degrees and a distance of 1783 feet
6 to 1 results in 9.6 degrees and a distance of 1182 feet
The conclusion is with a C172 at least, an idle power approach at about 60/65 knots will result in these distances, each of which will keep you above the glide slope until you are at or below 200 feet if your aim point was the numbers. I didn't take the time to calculate when in fact you would dip below the glide path with these descent angles, obviously it would vary based on your actual glide angle. My guess would be at a glide angle of 5 degrees you wouldn't dip below till VASI until under 100 feet.
400A
New Member
Negative ghost rider. Draw out an airplane crossing the threshold and it's flight path. The flight path would be about 3 degrees. Break that into horizontal and vertical components. Too shallow a descent and you have more horizontal component, too steep and you have too much vertical. Too shallow equates longer ground roll, ie more engergy to dissipate, too steep and you have to flare higher to arrest the descent rate and thus float more chewing up more runway before you can get the weight on the wheels so the brakes can stop you.
The point isn't to nitpick performance details or lay claim to being the best pilot ever. The point is, those numbers you get from your POH or EFB, or dispatch are the result of a very accurate and calculated way the airplane will be flown. Deviating from that and those numbers are useless.
Driving home the bigger point, the reason people are "dragging" the airplane in, or landing on the numbers is because they want increased landing performance, when in fact, they have no way of knowing if they are actually getting better performance (shorter landing distance) or if they are simply getting the illusion of that. I would think, that if one really understood what your landing numbers meant, and everything was legal, there would be no reason to be creative in your operation of the aircraft.
In a perfect world, we as pilots shouldn't have to be performance engineer specialists, the book says X, we need X + whatever. To get X you do A B and C with the airplane. The FAA and the Regs are very clear about what you can and can't do with performance and what you need (121/135). If landing shallow, or landing on the numbers, etc were proven and reliable techniques, we would have procedures for them.
As much as we, myself included, like to bag on the FAA for being a government run entity, there are some fairly smart people at the helm on this stuff and I think we all could do well to heed their wisdom and play by the rules to the best of our abilities.
What do you mean hopefully the FAA would agree? If that was "the interpretation", then the FAA has spoken. Where is this interpretation?
-mini
Blackhawk said "follow vertical guidance to the DA/DH" while the FAA regulation says, "maintain an altitude at or above the glide path until a lower altitude is necessary for a safe landing." The FAA doesn't say anything about the DA/DH that I saw, is it listed somewhere else showing they agree?
Edit: Oh I am sorry I see the problem, I didn't say in the original post that it was blackhawks interpretation, my fault I changed that now.
400A: I am not really following you, ground roll doesn't depend on the flare. If you touch the wheels to the runway at 50 knots it will take less distance to stop than if you touch the wheels at 55. If you always touch at full stall (33 in a C172) than it wouldn't matter if you came in on a 1, 3 or 5 degree slope, as long as you got to stall speed before the wheels touched. What am I missing?
I believe the generic term is VGSI. Do an AIM search for "VGSI" (visual glide slope indicator).
I believe thats true (speaking off the cuff). I just see most pilots use "the VASI" as a slang term for any visual glide slope indicator.......although I've been hearing more of "the PAPI"s" as a generic slang term, even though the particular system they may be looking at or referring to might not necessarily be PAPIs.
I do that. Old-timer's disease is my excuse.
400A
New Member
Shdw
The "difference" is where on the pavement you actually put the wheels down. If you come in steep, you may be aiming at the 500 foot markers, thinking you will flare and touchdown on the 1000 foot markers. When you actually will probably flare high, float past the 1000 foot markers, then put the wheels down. You are correct, your ground roll once the wheels are on the pavement (if you come in "high") will probably be roughly the same, but the distance you chewed up by flaring just took away any advantage you were going for by coming in steep.
Clear as mud?
The "difference" is where on the pavement you actually put the wheels down. If you come in steep, you may be aiming at the 500 foot markers, thinking you will flare and touchdown on the 1000 foot markers. When you actually will probably flare high, float past the 1000 foot markers, then put the wheels down. You are correct, your ground roll once the wheels are on the pavement (if you come in "high") will probably be roughly the same, but the distance you chewed up by flaring just took away any advantage you were going for by coming in steep.
Clear as mud?
Or wheels. That will definitely ensure you can stop by the first turnoff..........taxi may be difficult.
-mini
It doesn't make scientific sense at all. In fact if one flares high, regardless of angle, at the same speed they flare low they will likely land sooner because they won't have ground effect issues and will burn speed faster. Can someone break the science down as to how approach path can lengthen ones landing distance? If 3 aircraft approach the same point, one at 1 degree, one at 3 degrees and one at 5 degrees, each with the exact same weight, exact same conditions/configuration, and each at the same speed which will land shortest/longest and why?
I am not saying this is wrong, I don't know. The current conclusion proposed by 400A and agreed by mini is that the lower, 1 degree and higher, 5 degrees will all have a longer landing distance due to the flare with the steeper approach and some horizontal thing I am confused about making the shallower approach longer. I would expect each the distance to be shorter with a steeper glide angle, but only marginally.
tgrayson
New Member
shdw said:
The problem with such analyses is that you have to make assumptions before you can calculate anything. Based purely on the assumptions you make above, and further assuming that when you say “same point”, you mean only the distance along the ground rather than flare height, the following analysis applies.
The distance covered during the flare is
flare distance = Radius of loop * sin(approach angle).
The loop in question is the imaginary loop the pilot is performing during the pullup from an angle of descent to level flight. For instance, during a pullup from a 3 degree angle of descent, aircraft's flies 3 degrees of some circle as it changes to a landing attitude, and the portion of the circle looks like a slice of pie.
The radius of the loop is determined by the load factor and velocity of the aircraft. If you assume a constant load factor and velocity, the radius is constant and the above equation shows that the flare distance increases with an increasing approach angle.
That doesn't mean that a steeper angle always results in a longer total landing distance, because there are other variables at play, such as how many g's the pilot is willing to pull during the flare and how willing he is to change his airspeed based on his approach angle. And, there's no reason to think that 3 degrees provides the shortest distance; that angle is probably more geared towards a safe approach rather than a short landing.
My bet for the shortest landing would be a steep approach followed by a higher-than-normal load factor flare. The reasoning is that the loop you're going to fly has a smaller radius with more g's and you can go from your steep angle to level flight in a shorter horizontal distance. You probably wouldn't bleed off much airspeed in the flare so your touchdown might be a bit fast, but a slightly slower approach speed would fix that. Regardless, brakes are more effective than aerodynamic drag.
(The source for the above analysis is "Aircraft Performance and Design", by John D. Anderson. However, the conclusions are mine.)