View Full Version : PAPIs, VASIs and Glide Slopes

1st Sep 2001, 13:15
Could someone give me a very good answer on this.

If a PAPI was calibrated to a 3 degree slope, would maintaining 3reds 1white make you land shorter or on the TDZ if you didn't flare(just for thoretical purposes)

Would you still be flying a 3 degree glide path if you maintain 3R 1W.

Is it the same on a VASI and the glide slope of an ILS.

If it brings you to the TDZ, then being 3whites on the PAPI shouldn't be problem since you will stll be ending up on the TDZ. Only your rate of descent will be higher.

Also, if I am 3reds and maintain a normal 3degree rate of descent, would I end up being 4reds. ANd the same for 3whites, would i end up being 4 whites.


2nd Sep 2001, 11:35
PAPI and Glide Slope are point source aides, so flying "one dot high" on either will still get you to the touch down zone, albeit at a higher rate of decent.

T-VASIS in Australia is a multi-cource aid, flying "one dot high" still brings you in on a 3 slope, but with a higher threshold crossing height and a displaced touch down zone.

2nd Sep 2001, 13:07
Not quite correct, Checkboard.

With (as the qwuestion poses) three reds and one white you'll be coming in at less than 3 degrees (if that's what they're set for).

You can't make an assumption about your rate of descent. The angle at which you're coming in is related to your groundspeed. If you maintain 3 degree glide, a higher groundspeed equals a higher rate of descent. Maintain less than three degrees with the same groundspeed and your rate of descent is lower.

And finally, since there is a spatial difference between the pilot's eye and the undercarriage, if you maintain less than three degrees, the pilot's head is lower, therefore the gear is lower, therefore you WILL touch down shorter.

2nd Sep 2001, 15:06
That's what the question posed - not what I posted! ;)

I accept the correction though - must have been rushed, eh? :D

3rd Sep 2001, 00:49
If a PAPI was calibrated to a 3 degree slope, would maintaining 3reds 1white make you land shorter or on the TDZ if you didn't flare(just for thoretical purposes)I hope this one is answered
Would you still be flying a 3 degree glide path if you maintain 3R 1W.Nope, maintaining that would give you a flatter approach.
Is it the same on a VASI and the glide slope of an ILS.ILS is the same principle, yes. VASIs, since they are at two locations, one short of TDX and one long, complicate the geometry somewhat, but once you get two reds there is no predicting at all where you are and what you're doing.
If it brings you to the TDZ, then being 3whites on the PAPI shouldn't be problem since you will stll be ending up on the TDZ. Only your rate of descent will be higher.Correct, for the same airspeed.
Also, if I am 3reds and maintain a normal 3degree rate of descent, would I end up being 4reds.Yes, you would
ANd the same for 3whites, would i end up being 4 whites.Yes

A7E Driver
3rd Sep 2001, 01:17
During landings on the aircraft carrier, if you flew a consistently high approach (one ball/dot high), you would inevitably land short catching the first wire (not good) ---- as you would be underpowered with a high rate of descent and would not be able to "catch it" when you finally saw the "ball" (glide slope indicator) begin to descend in close.

Likewise, if you flew a slightly low approach, you would inevitably "bolter" (land long --- missing the wires) as you would be overpowered in close, too flat, and not be able to catch the rising glide slope at the last second.

3rd Sep 2001, 07:44
On a PAPI, flying a different light sequence will cause you to fly a steeper (1 red, 3 white) or shallower (3 red, 1 white) glide slope, because the lights are all in the same plane.

Flying a low (1 dot low) ILS will be the same -- you'll be shallower.

On a 3-bar VASI, you can fly the same 3 deg glideslope on either the top 2 bars or the bottom 2 bars, because they are set in different planes.

The key to the magnitude of the problem is the difference between the height of your eyeball and the height of the first piece of airplane to hit the pavement! If you maintain a constant PAPI or ILS indication, your eyeballs (or the ILS antenna) will be on a direct vector for the same piece of pavement. The metal and rubber hanging beneath them will tend to hit progressively shorter as you get progressively lower and/or shallower.

If you're flying a Cessna 150, it just doesn't matter! With a low but constant PAPI/ILS, you'll still land in the same area, and maybe bounce a little less...

If you're flying a B-747, the main body gear are hanging about 45 or 50 feet below your eyeballs. If you fly a 3 red/1 white PAPI, you will be low AND shallow, causing you to touch down well short.

Static Discharge didn't fly a constant one-ball-high approach when he got the 1-wire. He was trying to bring the ball to center, setting up an even higher rate of descent than he would have had flying the constant high ball. A constant 1 ball high would have gotten him a consistent 4-wire (3-wire on the Midway) with an occasional bolter.

A7E Driver
3rd Sep 2001, 12:46
I take your point Intruder --- If you can hold the slope one high or low without correcting to center, that will dictate where you land.

On the other hand, I will stick with my belief that the usual consequence of a high approach is a short landing --- and the usual consequence of a low approach is a long landing.

BTW A7/F18.

4th Sep 2001, 09:39
I know that coming in shallow will definitely land you deeper into the runway i.e. With all the above arguments, coming in shallow means more power to maintain the speed and your Rate of descent is lower. At flaring you would have a lesser sink rate and more power. Wouldn't you tend to float a little and land deep.

4th Sep 2001, 10:07
The point of the question was that you DIDN'T flare.

4th Sep 2001, 22:02
My mistake Hugmonster. I should have added the next question to that i.e "So, there would be no purpose coming in low since you would still end up at the TDZ" I have had pilots tell me to dip below the PAPI glide path so as to land short on short runways. WOuld this reasoning justify flying 3Reds

5th Sep 2001, 01:19
There are no hard and fast rules on that one. In a light single, there is very little wrong at all with coming in on three reds.

However, if we're really going to look at different techniques, you need to examine the detail of the aircraft.

How much power does it normally need?
High wing or low wing?
What type of flaps? (Fowler, split, whatever?)
How much flap for the wind conditions?
Tricycle undercarriage or taildragger?
Conventional, V, mid- or hight-tail?
Are you likely to have to go around?
Where's your C of G today?
What weight are you today?

etc etc etc.

Larger aircraft are flown completely differently to small ones, due mainly to a much higher inertia. Basically, you control speed with power and rate of descent with pitch, unlike a conventional light single where you should set rate of descent with power, and speed with pitch. Also, where runway length is rather more important with large aircraft, you should NEVER attempt a "greaser", for two reasons. One is that you use up far too much runway getting it down smoothly - just thump it on if you have to, and get it slowed down. The other is the length of the fuselage compared to its height off the ground - a greaser of a landing is a VERY small difference in deck angle to a tailstrike.

Chimbu chuckles
5th Sep 2001, 07:04
Flying a 'one dot low' approach on a PAPI will bring you in on a slightly lower/flatter approach to the same point on the runway because all the lights are in the same plane. With the undercarriage behind and below they will touch down closer to the threshold than on a normal 2 red/2white approach,presuming no 'flare'.
With a T-VASI flying one dot low(upright 'T') you'll be coming down a similar approach angle to a point closer to the threshold so will touchdown 'short'.
The point of flying a one dot low approach onto wet/short runways is to ensure touch down at the correct place AFTER you flare,IMHO. In other words the PAPI/VASIS approach plane intersects the flightstrip at a specific point(the 1000' markers)if you're on slope, having rounded out and flared the aircraft to the correct attitude for touch down you'll touchdown beyond that point(say 1200'-1400' in). Flying a 'low' approach simply brings the touchdown point back towards the 1000' markers, if done correctly.

Can anybody comfirm/deny something I vaguely remember hearing/reading many moons ago that visual approach aids are 'calibrated' to give a specific threshold crossing height for a specific size of aircraft? Say a 737 or 727 ? If not you would think that the same height of eye on a 747(assuming on slope) would bring the dunlops perilously close the threshold as opposed to the touchdown zone. I vaguely remember hearing that 747s make a 'one dot high' approach for this reason.


5th Sep 2001, 10:49
There maybe some truth in your question Chuck. Here in Malaysia, the international airports' PAPIs are caliberated for a wide body jet. Meaning a B737 would be seeing 3reds even if it were on a 3 degree glide.

6th Sep 2001, 00:45
First, and especially for those who fly into NCE, remember that the PAPIs are a piece of precision optical kit mounted right next to a bit of concrete onto which people keep dropping huge aeroplanes. Therefore, just as it is true that an ILS GS is (in some cases very) far from being a straight plane in space, so it is true that keeping the PAPIs exactly in kilter is close to impossible.

Now, here are my three questions:

(i) What's the point of discussing this without considering the flare?

(ii) What is the practical objective achieved by flying deliberately off the PAPI indication?

(iii) How about the effect on touchdown zone scatter, landing performance, and engine spool-up times, pitch rates, body contact criteria, etc., for the missed approach, if you don't follow the visual glideslope?

7th Sep 2001, 00:45
As I see it, the major problem is that we're not told about the set up in the aerodrome booklets.
OK, so we know what the PAPI 'on slope' angle is set at, but we're not told where the PAPIs are located in relation to the glideslope aerial. Many airfileds(but which ones??) have the PAPIs set up 120m upwind of the glideslope aerial - so you will/ you must get at least three reds if you maintain the ILS glideslope.
The aiming point reigns supreme.
But it would be nice to be told.
However, aren't visual slope aids purely calibrated for Cat I minima, therefore not to be relied upon below approx 200R?

8th Sep 2001, 19:15

(i) Because it is an easy way to talk about GPI or Ground Point of Impact, ie where that particular optical glide path intersects the runway.

(ii) It has a significant bearing on the TCH (Threshold Crossing Height) of your "dangly bits" - dangling is more proper for those who care.

(iii) They are all effects - the original question was constructed (I think) to avoid all of that.

9th Sep 2001, 22:52

...and my point is that, as soon as you start moving away from the 'normalised' way of operating, you achieve several things...

(i) You're out on your own;

(ii) You have no means of monitoring 'your' method;

(iii) You'll become a well-known character on the line and people will feel uncertain of you.

..and more.

If someone has found a better way of setting up the systems we use, he should put it forward for evaluation, through the company, the manufacturer, or the regulator, or whoever. If not, sticking with what we know won't kill us.

Performance questions affect the bottom lne of airline acounts. If the new way is better, the company will make more money. FACT.

Until the new way is the standard way, it's best to do it the way everyone else does.....

10th Sep 2001, 17:13
NorthernSky, I don't think anyone was suggesting trying to land without flaring, nor in deliberately going low on the approach.

I took the question as being a matter of academic interest in the geometry of an approach, not a question of landing or approach technique.

I hope that calms your fears.

10th Sep 2001, 18:05
Two white/two red give a 3 degree path. If you go to one white and three red, that is a 2.8 degree path with most PAPI, VASI installations. The 3 degree standard installation should give you a visual height of no less than 25 feet above the threshold. Some give you more depending upon the airport requirements.

Is there a gain going to three red? At proper approach airspeed, it will give you a touchdown approximately 500 feet short of the touchdown blocks, or about 30 feet over the threshold.

15th Sep 2001, 14:50
IMHO..At the end of a long duty... night and dawn landing accidents generally occur.
I feel Captains should never be allowed to forget that if they have autoland available, and there is a choice of autoland runway available that is within autoland limits, they should not be in such a hurry or too rushed or proud to use autoland.
Please correct me if I am wrong but I donot believe anyone has yet died during a crash on an actual or practice autoland.
Gulf Air GF072 crew and passengers (143) RIP.

[ 15 September 2001: Message edited by: scanscanscan ]