flexo
Topic Author
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### Landing (Stopping) Distance Of Small Aircraft

The other day I was reading a car magazine and noticed that most modern automobiles can come to a full stop within a distance of about 40m (~120 feet) when going 100 km/h (60 mph).

Now since that is about the landing speed of small aircraft (C172, C150, etc.) I was wondering why those need almost ten times the distance to come to a stop (~300m / 1000 feet)?
Since they have wheel brakes like cars and weigh about the same should it not be possible to stop within a similar distance, or are the official minimum runway lengths always stated for the worst case scenario (i.e. wet / slippery runway) and therefore in optimal conditions you can stop way faster?

Jetlagged
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### RE: Landing (Stopping) Distance Of Small Aircraft

The wheel diameter of a car is larger and so the brake discs are larger and more powerful. Tyre contact area of a car is correspondingly larger too. Cars have four wheel braking, light aircraft two wheels only.

I suppose the aircraft brakes could be made more powerful than they are, but that would add weight for no real gain as there aren't many 100 m tarmac runways about.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

jamesbuk
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### RE: Landing (Stopping) Distance Of Small Aircraft

Also could it be because the aircraft brakes aren't as effective straight away? Because the aircraft is still generating lift so not all the weight is on the wheels.

Just an idea I'm throwing out, not sure if its right.

Rgds --James--
You cant have your cake and eat it... What the hells the point in having it then!!!

bok269
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### RE: Landing (Stopping) Distance Of Small Aircraft

An educated guess...

Remember that cars propel themselves forward by pushing against the ground with the wheels. Airplanes propel themselves through the air by pulling/pushing against the air. As a result, when on the ground, the aircraft moves as a result of the plane pulling itself through the air. The tires move while taxiing or while landing because the airplane is pulling itself through the air, but the weight is on the tires. Now, when you brake a car, you are slowing it's direct source of forward movement-the wheels. Automobile brakes work by putting pressure on a disc or a drum that rotates with the wheel. Pressure is put against that and it works against the wheel turning. Now in an airplane, braking is done against the wheel (I am not familiar with the exact setup). However, the plane isn't moving as a result of the wheel turning. Its moving due to the propeller's pulling the plane forward against the air. When you hit the brakes, you are putting pressure against the wheel. This doesn't stop the plane's forward momentum because the plane's forward momentum was the result of the entire flight's worth of propulsion. While the propellor is at idle, the plane still has a lot of forward momentum. Stopping the wheels just makes it harder for that forward momentum to pull the plane forward. Eventually, it becomes too much, and the momentum can not pull the plane any further.
"Reality is wrong, dreams are for real." -Tupac

flexo
Topic Author
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### RE: Landing (Stopping) Distance Of Small Aircraft

Thanks for the replies!

So I guess there are several reasons why planes take longer to stop than cars:
- Continuous forward momentum applied by prop at idle
- Continuous lift generation by the wings resulting in less weight on the ground
- Shorter stopping distances are just not needed therefore the extra weight of bigger wheels / brakes is avoided

So as an extra question: What would be the absolute minumum distance (at optimal conditions) a C172 could stop? Anyone tried it?

Jetlagged
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Bok269 (Reply 3):An educated guess...

A few problems with your theory. Car brakes and aircraft brakes work the same way. Both put a torque on the wheel, via discs or drums, which slows the vehicle down. It makes no difference whether the wheel is driven or not. Also an idling propeller is providing quite a lot of drag, especially if it has a constant speed unit. Finally momentum is nothing to do with thrust. It is simply mass times velocity.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

SEPilot
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### RE: Landing (Stopping) Distance Of Small Aircraft

Aircraft brakes are not designed for panic stops; the above posts explain why they wouldn't be able to in any case. At the time an aircraft touches down little weight is on the wheels; and consequently little braking power is available. What would be the point in any case? The runway has to be at least that long for the plane to take off anyway. In fact, most small planes are capable of landing in a shorter distance than they can take off in in any case.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler

bok269
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Joined: Fri May 04, 2007 10:19 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Flexo (Reply 4):Continuous forward momentum applied by prop at idle

Not quite what I was getting at...let me give you an analogy [looks over at dog sleeping on the couch]. When you walk a dog, you act like their brakes. You stop them from running away, slobbering over small children, etc. For argument's sake, let's say you can stop a dog one of two ways: you can hold the leash firm, or you can physically hold their legs so they can't move (not that you would, but bare with me). When you physically hold their legs back, you stop their means of using the object that delivers their energy to the ground: their legs. Thats how automotive brakes work. They stop the wheels, which use the energy to push against the ground. When you pull on the leash, you are acting like aircraft brakes; you aren't stopping the actual propulsion system, but you are giving it more resistance that it will have to overcome.
"Reality is wrong, dreams are for real." -Tupac

IFACN
Posts: 149
Joined: Sun Nov 20, 2005 7:14 am

### RE: Landing (Stopping) Distance Of Small Aircraft

Landing roll, on a plane, is influenced by many factors:
- weight
- air temperature/runway elevation (density altitude)
- speed at touchdown (is influenced by the preceding factors plus crosswind/gust component)
- flap setting (influences touchdown speed)
- runway slope

The C172M (1043kg max landing weight) at 15 Celsius at sea level stops in 155 meters (landing distance is 381 meters).
For temperatures above 15C, add 10% to those distances for each +15 tranche (+10% at 30C, +20% at 45C, etc.).

A.

KELPkid
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### RE: Landing (Stopping) Distance Of Small Aircraft

I used to rent a Cessna 172 with the Horton STOL Craft kit on it. One day, I asked an instructor to show me how to take advantage of the STOL (Short Takeoff and Landing) features. On my first STOL landing, he told me to aim for the piano keys (the runway end markers). We made the first taxiway turnoff that time   , the one that you'd normally use to enter the runway for takeoff.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

KELPkid
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Flexo (Thread starter):The other day I was reading a car magazine and noticed that most modern automobiles can come to a full stop within a distance of about 40m (~120 feet) when going 100 km/h (60 mph).

A Cessna 172 is indicating about 65 knots or so on a typical landing...a knot is approx. 1.2 mph, so figure on about 78 mph groundspeed (assuming no wind).

 Quoting Flexo (Thread starter):Now since that is about the landing speed of small aircraft (C172, C150, etc.) I was wondering why those need almost ten times the distance to come to a stop (~300m / 1000 feet)?

Negative-see the computation above on the speed.

Also, notice how in a car, the minimum acceptable rim diameter seems to be about 14" or so (much larger for performance models, or even a little smaller for some Korean imports)? Well, a Cessna 172 has 6.00x6" mains (the 6.00 means the rim diameter is 6", so you can imagine what the brake disk size is compared to car...TINY!). Much much less braking area, and the tire has a tiny contact patch to boot.

Hey, at least in the plane, we can do one cool thing that cars can't: control the left and right brakes independently  (perfect for taxiing the plane into a tight parking spot).
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

Jetlagged
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Flexo (Reply 4):Continuous forward momentum applied by prop at idle

Propellers do not apply momentum.

 Quoting Bok269 (Reply 7):Thats how automotive brakes work. They stop the wheels, which use the energy to push against the ground. When you pull on the leash, you are acting like aircraft brakes; you aren't stopping the actual propulsion system, but you are giving it more resistance that it will have to overcome.

That analogy only makes sense if you imagine the engine is providing forward thrust during the rollout. But it's at idle so with any significant forward speed is creating drag. Wheel brakes are the same, whether fitted to cars or aircraft, working in exactly the same way, via friction.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

SilverComet
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting KELPkid (Reply 9):

How does a STOL Craft Kit work? Is it some sort of feature you 'activate' whenever you want (like extra flap settings?) or is it some sort of permanent modification to the airframe/engine/wheels to alter the aerodynamic/mechanical properties?

KELPkid
Posts: 5247
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting SilverComet (Reply 12):is it some sort of permanent modification to the airframe/engine/wheels to alter the aerodynamic/mechanical properties?

The Horton STOL craft kit consists of: (on the 172 at least):

- A cuffed leading edge that droops "lower" than the original leading edge of the 172.

- Flap gap seals

- An aerodynamic fence between the ailerons and flaps on the top of the wing

- "Drooping" wingtips

 Quoting SilverComet (Reply 12):How does a STOL Craft Kit work?

It basically improves the low speed aerodyamics of the aircraft so that it's safely controllable at lower indicated airpeeds than it would otherwise be...when you're on the back side of the power curve with the STOL kit, the aircraft isn't mushing along and is more controllable at those speeds.

Here's a picture of the Horton STOL Craft kit (on a Cessna 150):

View Large View Medium

Photo © Adam Wright

Celebrating the birth of KELPkidJR on August 5, 2009 :-)

SlamClick
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### RE: Landing (Stopping) Distance Of Small Aircraft

Many published "landing distance" figures are from over a 50' obstacle. So even with a very steep approach a good deal of this distance is flown over, not rolled upon.

If we are talking actual landing roll - touchdown to full stop we have a more valid comparison.

Most published stopping distances for cars refer to "panic stops" where the driver is trying for the smallest possible number. Rarely is an airplane landed with such a purpose.

Believe me I have done a lot of STOL operations. Here are a few figures from memory:

DeHavilland Beaver - I routinely used a grass strip 333 feet long (101.5m) at a fairly high density altitude.

Helio Courier - I once landed in a sky diving target, a shallow pit filled with wood chips about 40 feet (10m) across. For regular ops, I like a thousand feet but never used more than 200-250 (61 to 76m) The rest is for clearing the trees.

Cessna 206 with STOL mod - I routinely took myself and five passengers in and out of 1000' (305m) dirt strips at elevations up around 7000 feet (2134m) I carried just enough fuel for the round trip and VFR reserves.

I've landed a stock Cessna 402B many, many times in a measured 500' (152m) but it does heat the brakes up pretty good.

As a student pilot I got a Cessna 150 stopped in 140 feet. (43m)

Non-STOL?

How about watching McDonnell-Douglas DC-10 test pilots do a "maximum energy stop" at maximum landing gross weight at KYUM. It rolled about 1000 feet (305m) at 363,500 pounds (164881kg)

Now how much distance do you think would be needed to stop a Mercedes sedan from something like 240 km per hour and at 164881kg using Mercedes sedan brakes? I'm guessing Europe might be too small for such a test.
Happiness is not seeing another trite Ste. Maarten photo all week long.

Ralgha
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### RE: Landing (Stopping) Distance Of Small Aircraft

I've stopped a C152 in about 100 feet.
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bok269
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Joined: Fri May 04, 2007 10:19 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Jetlagged (Reply 11): That analogy only makes sense if you imagine the engine is providing forward thrust during the rollout. But it's at idle so with any significant forward speed is creating drag. Wheel brakes are the same, whether fitted to cars or aircraft, working in exactly the same way, via friction.

I was thinking that would be the kink in my explaination. Thanks for clearing it up.
"Reality is wrong, dreams are for real." -Tupac

N231YE
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Joined: Tue Jul 25, 2006 9:24 am

### RE: Landing (Stopping) Distance Of Small Aircraft

This should help, from my PIM. Also note the the various items that must be factored in when computing the distances:

Disclaimer: this is from a Cessna 172R Pilot's Information Manual (PIM). This is also an internet site, therefore, this IS NOT to be used for realistic navigation or flying purposes

 Quoting KELPkid (Reply 13):- Flap gap seals

Don't you mean, aileron gap seals?

ejapilot
Posts: 6
Joined: Wed Aug 29, 2007 10:43 am

### RE: Landing (Stopping) Distance Of Small Aircraft

One other thing to consider, most single engine piston planes do not have antiskid, that will have an effect on the distance compared to a similar weight car with an ABS system,

2H4
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### RE: Landing (Stopping) Distance Of Small Aircraft

I'm surprised nobody has brought up the issue of weight distribution.

When a car stops, the weight shifts to the front wheels, and a great deal of force can be applied to those wheels before lockup becomes a factor.

When aircraft brakes are applied, the weight also shifts to the front wheel(s). Because the braking power is restricted to the rear wheels, however, lockup occurs much sooner, and the maximum braking force is much lower.

Think of it this way....grab a handful of rear brake on a bicycle, and the rear wheel will lock up and start skidding. Grab a handful of front brake, and your ride will quickly become very interesting.

In short, if you only have rear brakes, weight distribution (under braking) will greatly reduce their effectiveness.

2H4

Intentionally Left Blank

SlamClick
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting 2H4 (Reply 19):In short, if you only have rear brakes, weight distribution (under braking) will greatly reduce their effectiveness.

True, but an airplane still has some elevator authority well below flying speed. This can be, and is used to keep weight on the mains.
Happiness is not seeing another trite Ste. Maarten photo all week long.

SilverComet
Posts: 85
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting KELPkid (Reply 13):

Thanks for that explanation. Much appreciated.

 Quoting SlamClick (Reply 14):Many published "landing distance" figures are from over a 50' obstacle. So even with a very steep approach a good deal of this distance is flown over, not rolled upon.

I should think that this is the most contributing factor to the discrepancy in the figures.

KELPkid
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting SEPilot (Reply 6):Aircraft brakes are not designed for panic stops

Most definitely not. Any pilot doing a panic stop in a light aircraft will be in the market for new tires in a hurry, as GA aircraft tires flat spot (sometimes down to the chord!) rather easily

 Quoting N231YE (Reply 17):Don't you mean, aileron gap seals?

Nope, on the Horton kit it's flap gap seals. Besides, the ailierons on the 172 don't really have much of a gap between themselves and the wings when they're in trail. Heck, the piano hinge on the top of the wing between the ailieron and the wing practically is a gap seal   Maybe the other guy's STOL kit (Robertson I believe?) has ailieron gap seals...
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

N231YE
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting KELPkid (Reply 22): Maybe the other guy's STOL kit (Robertson I believe?) has ailieron gap seals...

That's the kit I must be thinking of.

 Quoting 2H4 (Reply 19):Grab a handful of front brake, and your ride will quickly become very interesting.

Been there, done that

 Quoting 2H4 (Reply 19):

To shorten one's landing distance in a 172, one must pull back on the yoke and reduce the flaps, in addition to maximum braking. By doing so, at least one can control some of the weight distribution to the main gear wheels.

vzlet
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### RE: Landing (Stopping) Distance Of Small Aircraft

If one were to land a plane on a conveyor belt whose upper surface was in motion at a speed matching the plane's, but in the opposite direction, the plane would come to an instant halt with no ground run. (   Well, at least according to numerous posts in this infamous thread.)
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid

BAE146QT
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Vzlet (Reply 24):If one were to land a plane on a conveyor belt whose upper surface was in motion at a speed matching the plane's, but in the opposite direction, the plane would come to an instant halt with no ground run.

I assume you're joking, but inertia would carry the aircraft forward until the point where the tyres could get sufficient grip, and the brakes became effective. At which point I imagine your forehead would contact the yoke (or the seat back in front of you).
Todos mis dominÃ³s son totalmente pegajosos

Starlionblue
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Bok269 (Reply 3):When you hit the brakes, you are putting pressure against the wheel. This doesn't stop the plane's forward momentum because the plane's forward momentum was the result of the entire flight's worth of propulsion. While the propellor is at idle, the plane still has a lot of forward momentum. Stopping the wheels just makes it harder for that forward momentum to pull the plane forward. Eventually, it becomes too much, and the momentum can not pull the plane any further.

This is not right. The physics of stopping an aircraft with wheel brakes and a car with wheel brakes are the same.

 Quoting SlamClick (Reply 14):Helio Courier - I once landed in a sky diving target, a shallow pit filled with wood chips about 40 feet (10m) across.

Pray tell us more about this incident.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

SEPilot
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Joined: Sat Dec 30, 2006 10:21 pm

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Vzlet (Reply 24):( Wink Well, at least according to numerous posts in this infamous thread.)

That's why I have studiously avoided that thread. I'm afraid that such ignorance might be contagious.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler

lowrider
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Joined: Wed Jun 30, 2004 3:09 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Flexo (Reply 4):So as an extra question: What would be the absolute minumum distance (at optimal conditions) a C172 could stop? Anyone tried it?

With a little headwind, I was easily able to achieve a stopping distance of 200 feet. Part of a reason for the discrepincy between cars and light planes is that the brakes on light planes are simply older technology. For example, most light aircraft do not use boosted brakes. What ever pressure you are able to apply to the pedals it what gets trasmitted to the brakes, as opposed to the "power brakes" set up on most cars. Also, light aircraft brakes are a balance of cost, weight, size, and simplicity. On a light aircraft, it is simply not vital to extract every foot pound of stopping power out of the brakes. Many of them can already get into fields they cannot get out of.
Proud OOTSK member

bok269
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Starlionblue (Reply 26): This is not right. The physics of stopping an aircraft with wheel brakes and a car with wheel brakes are the same.

So I've been made aware. I stand corrected.
"Reality is wrong, dreams are for real." -Tupac

bok269
Posts: 1568
Joined: Fri May 04, 2007 10:19 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Starlionblue (Reply 26): This is not right. The physics of stopping an aircraft with wheel brakes and a car with wheel brakes are the same.

So I've been made aware.   I stand corrected.
"Reality is wrong, dreams are for real." -Tupac

BAE146QT
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### RE: Landing (Stopping) Distance Of Small Aircraft

Bok269 - I hope I didn't come across as snippy. I should have put in a smiley.
Todos mis dominÃ³s son totalmente pegajosos

Starlionblue
Posts: 18084
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Bok269 (Reply 30):Quoting Starlionblue (Reply 26): This is not right. The physics of stopping an aircraft with wheel brakes and a car with wheel brakes are the same. So I've been made aware. Wink I stand corrected.

I have stood corrected many times on these boards. It's always a learning experience.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Ralgha
Posts: 1589
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### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Lowrider (Reply 28):What ever pressure you are able to apply to the pedals it what gets trasmitted to the brakes, as opposed to the "power brakes" set up on most cars.

They don't need boosted brakes, it's quite easy to lock up the wheels on a light airplane already. One thing that does make a difference is the lack of an anti-skid system.
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Ralgha (Reply 33):One thing that does make a difference is the lack of an anti-skid system.

Another (as has been mentioned above) is the incredibly thin tires on those itty-bitty airplanes. They are incredibly easy to scalp. (Maybe from being mounted on Cherokees and Comanches) (kidding!! no offense intended to the worthy people of those nations)
Happiness is not seeing another trite Ste. Maarten photo all week long.

lowrider
Posts: 2542
Joined: Wed Jun 30, 2004 3:09 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Ralgha (Reply 33):They don't need boosted brakes,

I wasn't arguing that they did, I was merely pointing out some of the differences between the two, particularly in level of applied technology. Those little tires can only generate so much friction. As previously noted, it is easy to exceed that level with current technology in use.
Proud OOTSK member

bok269
Posts: 1568
Joined: Fri May 04, 2007 10:19 am

### RE: Landing (Stopping) Distance Of Small Aircraft

 Quoting Starlionblue (Reply 32): I have stood corrected many times on these boards. It's always a learning experience.

Amen!

 Quoting BAe146QT (Reply 31):Bok269 - I hope I didn't come across as snippy. I should have put in a smiley.

Not at all!
"Reality is wrong, dreams are for real." -Tupac

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