When did aircraft begin using systematic glide paths?

  • #1
Stephen Tashi
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From watching internet videos about air crash investigations, I gather that modern airliners follow a set "glide path" as they land. When, in the history of aircraft, did such a procedure begin? For example, did the large planes of WW2 use printed documents or radio beacons that defined a glide path?
 

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  • #2
russ_watters
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From watching internet videos about air crash investigations, I gather that modern airliners follow a set "glide path" as they land. When, in the history of aircraft, did such a procedure begin? For example, did the large planes of WW2 use printed documents or radio beacons that defined a glide path?
I'm not clear what you are asking. Are you asking about radio navigation aids or about airplanes landing in general? Because as phrased it sounds like you are asking about landing in general. Obviously if you are landing you have to descend, and a stable glide path is easier than one that is changing all the time. I'd think Orville and Wlibur figured that out pretty quickly.

This looks like it may have been the first radio navigation aid for landing (1932):
https://en.wikipedia.org/wiki/Lorenz_beam
 
  • #3
anorlunda
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Wikipedia says that the first tests of ILS (Instrument Landing System) began in 1929. That is slightly earlier than 1932, the date of the Lorenz beam that @russ_watters linked.
 
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  • #4
Stephen Tashi
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Wikipedia says that the first tests of ILS (Instrument Landing System) began in 1929. That is slightly earlier than 1932, the date of the Lorenz beam that @russ_watters linked.
According to that article, systems that give the information needed to follow a prescribed glide slope ( both distance and altitude information) were developed after WW2.

It seems possible that glide slope charts could have been used during WW2 by pilots making non-instrument landings. (Modern airliners may begin following a prescribed glide slope long before the pilots can see the airport.) Presumably WW2 pilots could know their altitude from their onboard instruments. I don't know if they could know the precise distance to the runway - or if they needed to know it before seeing the airport.
 
  • #5
Klystron
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According to that article, systems that give the information needed to follow a prescribed glide slope ( both distance and altitude information) were developed after WW2.

It seems possible that glide slope charts could have been used during WW2 by pilots making non-instrument landings. (Modern airliners may begin following a prescribed glide slope long before the pilots can see the airport.) Presumably WW2 pilots could know their altitude from their onboard instruments. I don't know if they could know the precise distance to the runway - or if they needed to know it before seeing the airport.
I have read several reports from WWII naval aviators using a system of lights and lenses to assist with night landing; on runways on land and at sea on a moving aircraft carrier. IMS one method employed Fresnel lenses and lights controlled by a landing officer (LO) stationed on a platform near the beginning of the landing zone. The aviator navigated to the airbase/carrier using printed maps and charts* following radio beacons then switched to voice comms before entering guide slope.

Given estimated aircraft weight and other characteristics and local weather data, the aviator was instructed to intersect the landing system lights at a certain altitude by the LO and 'follow the ball' of light down the guide slope to correctly intersect the mechanical arresting system on the flight deck/runway. The lenses were adjusted so that the light ("ball") indicated the optimum altitude to maintain proper guide slope.
 
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  • #6
anorlunda
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I have read several reports from WWII naval aviators using a system of lights and lenses to assist with night landing
That's called VASI, and it is used almost everywhere for a "visual" approach. ILS is used in bad weather or when the runway is still too far away to see.
1628101112911.png
 
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  • #7
russ_watters
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According to that article, systems that give the information needed to follow a prescribed glide slope ( both distance and altitude information)..
Where exactly did you get that idea? It's wrong, as the sources indicate. A glideslope is just a line projected into space by radio or light (often more than one, so you can tell if you are high or low). It does not use or require distance measurement.
It seems possible that glide slope charts could have been used during WW2 by pilots making non-instrument landings. (Modern airliners may begin following a prescribed glide slope long before the pilots can see the airport.) Presumably WW2 pilots could know their altitude from their onboard instruments. I don't know if they could know the precise distance to the runway - or if they needed to know it before seeing the airport.
You're doing a very poor job of speculating here and I feel like you have some big misunderstandings of airplane/flight operations. For starters, planes don't usually flight straight-in approaches from a very long distance. Descent and landing are two separate/distinct phases of flight and you might be mixing them together. Knowing when to start your descent (and how rapid it should be) requires knowing your distance from the airport, altitude and speed. Final approach and landing follows a pre-planned and practiced traffic pattern that doesn't require specific measurement/estimation of distance. While it's nice to be provided with a glideslope (and most airports do have them), it isn't required.

The idea of a pre-planned and practiced traffic pattern around an airport almost certainly originated from the Wright brothers, as then need and value becomes obvious the first time you decide to practice landings. You take off, make four left turns and then land again. Being able to fly that pattern exactly the same, repeatedly, is a basic/critical pilot skill. That includes following a prescribed glide path, whether it is marked for you or not.
 
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  • #8
Borek
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It does not use or require distance measurement.

Technically it doesn't, but to follow a path that is at a correct angle to the ground is geometrically equivalent to knowing both height and distance (plane drops by prescribed height per prescribed distance of ground covered). I think what you are both saying is in a way equivalent, even if the technical realization is what it is.

No idea about Wright brothers (that is, I agree they most likely followed some routine), but definitely around 1920, when the first ATC started to work at the Croydon Airport in London, they must have defined some rules, and approach path would be one of them.
 
  • #9
russ_watters
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Technically it doesn't, but to follow a path that is at a correct angle to the ground is geometrically equivalent to knowing both height and distance (plane drops by prescribed height per prescribed distance of ground covered). I think what you are both saying is in a way equivalent, even if the technical realization is what it is.
To me, saying it is "geometrically equivalent" to a distance and rate of descent is very different from in-practice knowing your distance and calculating or looking up in a table what the altitude and rate should be. To me it sounds like the OP is asking "when did pilots/planes get the ability to measure distance from the airport, which they need to use/ensure they are on the glideslope?" That's why my answer is "that's now how a glideslope works".

For a descent to an airport you do indeed calculate where you should start your descent based on how much altitude you need to lose and your speed/rate of descent. For final approach and landing you do not.

It is true that the exactly duplicated traffic pattern has pre-calculated/planned distance and slope, but you're only eyeballing-it, and it can change with wind or traffic.

Say for example you're flying downwind, parallel to the airport and someone is in front of you. You wait for them to pass the other direction on their final approach before making your left turn onto base leg. Ordinarily you'd start descending to lose 200' before making that turn, but you figure you are too far and skip that. Somewhere on base you look out the window and see where the airport is vertically (the angle) and guess you're near the glideslope and start your descent (maybe you can't see the VASI/PAPI or it doesn't have one). When you turn final, you literally see where the airport is on the windshield and adjust your angle/rate of descent to compensate. Say you're below glideslope because you extended pretty far; you slow your rate of descent and watch as the runway moves down on your windscreen. That's how "seat of your pants" it is.
 
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  • #10
jrmichler
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@russ_watters very nicely explains the techniques for flying Visual Flight Rules (VFR). Instrument Flight Rules (IFR) flying requires more instrumentation and more training. Modern IFR landing approaches are almost all based on GPS. The approach plate below shows a typical approach. It's for Runway 1 at my home airport.
KPBH RNAV1.jpg

The Cessna 172 I fly is category A in the table at the bottom left, while the local company jets flying in this afternoon are Category D. The C-172 has a GPS with LNAV capability, so the IFR approach procedure is as follows:

Fly to URPOW at an altitude above 4000 feet and below 6000 feet.
After passing URPOW, start descending so as to pass GOFXO at above 3100 feet.
After passing GOFXO, continue descending so as to pass CAMEM at above 2100 feet.
After passing CAMEM, descend to just above 1920 feet, and look for the runway. If you do not see the runway in time to make a safe landing, go around, and fly to a different airport.

There is a needle for horizontal guidance. If the needle is to the left of center, turn left.

Other airplanes have GPS with vertical guidance capability. Those systems have an additional needle for vertical guidance. Those airplanes can fly to LPV minimums, which are 1730 feet at this runway. A typical panel indicator is shown:
Indicator.jpg

When using vertical guidance, the airplane follows a straight path. When using only horizontal guidance, the airplane will either descend in a series of steps or in a straight path, depending on the skill of the pilot.

Later today I have to go to the airport and move the C-172 out of the hangar so they can park the two jets in there tonight.
 
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  • #11
anorlunda
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There's no real disagreement here IMO. @jrmichler showed us an approach plate for instrument landings. @russ_watters I think was describing the "standard pattern" used for visual approaches, as shown in the picture below.

In the pattern, the final descent is started on the downwind leg. The pilot configures the plane for landing and establishes his standard rate of descent.

The issue of glideslopes, ILS radio aids, or VASI aids do not become relevant until the relatively short "final" leg of the pattern. But before reaching final, the aircraft should be stable on the standard angle of descent, and distance, and altitude.

Then on final, the VASI or ILS can be used to confirm that you are doing it right, not to establish the correct parameters.

Note that even commercial airliners regularly switch from instrument procedures to visual procedures when they have the runway in sight "Clear for the visual," is a frequently heard phrase from the air traffic controllers.


1628164555771.png
 
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  • #12
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There is an aspect to the OP question someone might help me understand that I never have really understood.

The "standard glide slope" requires (in most 'planes) a powered approach. This seems a perfect opportunity for an inopportune engine failure and to crash before you make it to the landing strip, even though you might have had plenty enough altitude just a few moments ago to glide in.

Sure, engine failure on finals doesn't happen often, but why not follow the natural glide slope of a given 'plane? I mean, you have to roll out to a low, almost zero angle at some point (else it's a crash landing!), but why fix one's approach angle quite so early on a standard approach?
 
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  • #13
anorlunda
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but why not follow the natural glide slope of a given 'plane?
Not a bad question. If you "play safe" by coming in at a higher altitude, then you need to make up for it by nosing down at the last minute. That increases speed beyond what you want. Another alternative is to use slip, but that makes passengers nervous.

A multi engine aircraft can maintain the normal 3 degree slope with a single engine failure.

A single engine aircraft becomes a glider with the single engine out. But the single engine approach speed is very close to the best glide angle anyhow. If the engine fails on final approach, the pilot should use the power off landing procedure. Here it is from the Cessna 152 manual.
1628202231929.png


Note that the loss of power on final is compensated by using less flaps.

Pilots are trained to think about cases like engine failure on final. Procedures are established. Professional pilots practice it in the simulator.
 
  • #14
anorlunda
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Coincidentally, the following video was posted just minutes ago. It includes an interesting discussion about stabilization before landing. Even non pilots can follow what is said.

 
  • #15
russ_watters
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Additional thoughts on that:
  • If your throttle is already idle, you can't reduce power any further to adjust your descent rate. Slipping is about your only option. That makes power off landings much harder than powered landings.
  • A power off capable pattern will be steep, tight and awkward.
  • An engine that has already proven operable for the duration of a flight is unlikely to fail while at low power. Unless it runs out of fuel, which the pilot should already be following/aware of/making adjustments for.
 
  • #16
anorlunda
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I explained myself badly in #13. It's simpler. Steeper approaches tend to make the airplane fly faster. On landing, speed can be more difficult to control than altitude.

Landing an airplane is an exercise in 10 dimensional space. You must arrive at a point in 3D space, with 3D velocity vectors within prescribed limits, and 3D acceleration vectors within prescribed limits. The 10th is time. You may have other planes in front of you and others behind you. Your landing clearance is only good for a window in time. All ten interact with each other.
 
  • #17
jrmichler
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From watching internet videos about air crash investigations, I gather that modern airliners follow a set "glide path" as they land. When, in the history of aircraft, did such a procedure begin? For example, did the large planes of WW2 use printed documents or radio beacons that defined a glide path?
Getting back to the OP.....

Ground Controlled Approaches (GCA) for descending to an airport were developed during WW2. Those used ground based radar to show the location and altitude of an aircraft to an operator who talked the pilot down by giving continuous directional instructions.

I don't have a good technical reference, but Arthur C. Clarke wrote an excellent historical fiction book on the subject based on his experiences developing the system. The title is Glide Path, and it is still available: https://www.amazon.com/dp/0743475313/?tag=pfamazon01-20. A recommended read.
 
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  • #18
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Not a bad question. If you "play safe" by coming in at a higher altitude, then you need to make up for it by nosing down at the last minute. That increases speed beyond what you want.

But I am not suggesting going beyond the speed you want, at any time.

The profile would be;
1) (say) flying along cruise speed straight and level on long finals, clean trim, let's say 250kt
2) At the appropriate decent point, you throttle back, maintaining altitude, until you reach the maximum flapless landing speed (in case everything goes pear shaped and that's the speed you end up at), let's say it's 150kt
3) Having slowed straight and level to 150kt, you pitch forward and follow the slope to sustain 150kt, down to the roll out point
4) At the appropriate roll out point, you pop the flaps and slow to normal landing speed, say 100kt, set the engine to fast idle (make sure it is working in case you need to go around), and land, feel free to use the engine for fine final adjustments but it is no longer a 'safety need' to have it working to get into the aerodrome
5) If you see you are short, just pop the flaps a few moments later, puts you further down the landing strip
6) If you are long, pop the flaps earlier

Not only can the engine 'not fail' in that profile, it is also less CO2 as you don't spend 4 miles dragging the flaps against the power of the engine.



Note that the loss of power on final is compensated by using less flaps.
Why not the other way around; deal with too much glide energy with more flaps?

Surely better to have more energy in the airframe than you need that you can get rid of with flaps and slats, than have too little energy and have to rely on an extra part (engine)?



Additional thoughts on that:
  • If your throttle is already idle, you can't reduce power any further to adjust your descent rate. Slipping is about your only option.
... or the flaps and slats, which is what they are there for, to slow the 'plane and to fly slower, when needed?

The reason I think the above question and proposed glide slope is part of the OPs question is that, surely, in the age old days of dodgy aeroengines, wouldn't pilots want to land like this? If they did, then what changed? When did it get converted into this 'you must power your engine against flaps' profile? I feel sure the early guys never flew landings like that, the ones that lived the longed relied the least on their engines!
 
  • #19
russ_watters
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The profile would be;
4) At the appropriate roll out point, you pop the flaps and slow to normal landing speed, say 100kt, set the engine to fast idle (make sure it is working in case you need to go around), and land, feel free to use the engine for fine final adjustments but it is no longer a 'safety need' to have it working to get into the aerodrome
5) If you see you are short, just pop the flaps a few moments later, puts you further down the landing strip
6) If you are long, pop the flaps earlier

Not only can the engine 'not fail' in that profile, it is also less CO2 as you don't spend 4 miles dragging the flaps against the power of the engine.

Why not the other way around; deal with too much glide energy with more flaps?

Surely better to have more energy in the airframe than you need that you can get rid of with flaps and slats, than have too little energy and have to rely on an extra part (engine)?

... or the flaps and slats, which is what they are there for, to slow the 'plane and to fly slower, when needed?
That's a lot more complicated than normal procedure, which makes it more difficult to repeat and sequence airplanes for approach and landing. The key to a good approach is consistency and stability. The more dynamic changes you makes, the more difficult it is. For the flaps thing, sure, you can put them in early to lose speed/altitude faster, but for a small plane that's just 3 notches of flaps, and then you're done. They are also fairly large changes in lift/drag, so not a very precise way to adjust speed/sink rate.

Also, the 'lower flaps right before flare' and 'lower flaps later if you are short' procedures contradict each other. And lowering all the flaps at once is only used in an emergency because it is a huge change in the flight characteristics, and doing so right before landing makes re-establishing a stable approach to landing difficult.
The reason I think the above question and proposed glide slope is part of the OPs question is that, surely, in the age old days of dodgy aeroengines, wouldn't pilots want to land like this? If they did, then what changed? When did it get converted into this 'you must power your engine against flaps' profile? I feel sure the early guys never flew landings like that, the ones that lived the longed relied the least on their engines!
Well, the problem with dodgy engines wouldn't be unique to landing, and it is telling to me that we brief four separate engine failure scenarios for takeoff and none for landing. 100 years ago or today, by far the bigger engine failure risk is at takeoff and that proportion wouldn't change over time (just the overall frequency).
 
  • #20
anorlunda
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Consider:
  • Stabilization: The idea that configurations and parameters for landing are established in advance. No sudden actions should be required in the last seconds. Doing it in advance allows time to double check that it was done correctly. Co-pilots and VASI indicators can trigger awareness that something may be wrong. (See the video in #14 about stabilization.)
  • The pilot's internal calibration of what 3 degrees "looks like" is another backup warning system. It is actually pretty easy to develop a sense in pilots that "I'm too high" or "I'm too low." The same too high too low calibrations apply to small airplanes, giant airliners, and even gliders.
  • Go around: The ability to rapidly change from landing strategy to emergency go around.
  • The long time required to spool up jet engines. The time required to raise or lower flaps.
  • Vertical wind shear, microbursts, cross winds, wake turbulence. Those are all external things that can rapidly change airspeed.
  • Stall avoidance on final. That killed a lot of old geezers. Stall avoidance is best characterized by the difference between airspeed and stall speed for the current flaps configuration.
  • The idea that any of us could invent a new concept of flying airplanes that has not been evaluated before.
 
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  • #21
Stephen Tashi
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This is the video motivated my question: (On my browser, I can't watch it on the forum. I have to select "Watch on YouTube".

Perhaps the correct terminology for my question is "approach path" instead of "glide path".
 
  • #22
anorlunda
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The theory in that video is that the pilot misread vital information from a piece of paper because his thumb covered the vital part as he held the paper. So it was a miscommunication. That really has nothing to do with the nature of the vital information miscommunicated.

The pilot could also misread instruments, or mishear verbal communications.

Modern aircraft also have radio altimeters to measure height above the ground, and they have verbal warnings "terrain terrain" "pull up" "pull up." The video clip did not mention those.

So I think your OP question was off the mark. It has nothing to do with glideslopes or approach plans. It's simple miscommunication.
 
  • #23
Stephen Tashi
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That really has nothing to do with the nature of the vital information miscommunicated.
Yes. However my question is about the history of such charts, not about categorizing the cause of that particular air crash. When did pilots began using such charts?
 
  • #24
anorlunda
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I found an answer for aeronautical charts in general, not for those specific approach charts.

Bear in mind that some approaches such as the one in this accident are primarily by terrain, not by aeronautics. The particular altitudes at specific distances are set by the height of the highest mountains nearby. Any airport surrounded by mountains would have a crucial need for pre-planned approaches and they would likely be the first to publish them.

http://escape-maps.com/escape_maps/...of_Air_Navigation_Charts_in_the_United_States
The Library Section of the Information Division of the Air Service, both units having been organized about 1920, was responsible for the issuance of these maps to the Army Air Service.
Have a look at this accident at St. Barts. The approach requires clearing the top of a hill, then a steep nose down the hillside to the runway, then a very short runway. In this video, the pilot didn't manage all that. Clearly, no pilot should try to fly to St. Barts without a pre-plan for how to approach the airport, so I expect the age of the first charts to be about the same as the age of the airport.

 
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  • #25
jrmichler
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The approach plate in Post #10 is roughly 20 year old GPS technology. The technology before that used VOR and ILS. VHF Omnidirectional Range (VOR) is a transmitter. The pilot sets the frequency, turns the OBS knob (bottom photo in Post #10) until the needle centers, then keeps the needle on center until over the VOR transmitter. The compass ring gives the bearing from the transmitter.

The ILS consists of a localizer and glideslope. The localizer is a transmitter next to the runway that transmits a signal that gives left/right indication down the the runway. The glideslope is a transmitter that gives up/down information down to the runway. While the rules allow flying the ILS down to an altitude of (typically) 200 feet, the ILS works well enough to put the airplane onto the runway without seeing outside. I did this in training while under the hood. Once, with an instructor.

This technology was mature when I got my instrument rating in 1975.

Further research found this: https://en.wikipedia.org/wiki/Instrument_landing_system. The original ILS was first invented in 1929, and developed into its modern form about 1945. I don't know when approach charts developed into their modern form, but they look the same as those of 1975. Here's the ILS approach to a nearby airport:
ILS9 RHI.jpg

A Cessna 172 can fly this approach down to 200 feet above ground in 1/2 mile visibility. At 200 feet and 1/2 mile, only the approach lights on the very end of the runway are visible when the pilot has to make the decision whether to land or to go around.
 
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