Menu Sign In Contact FAQ
Banner
Welcome to our forums

Is a hard landing necessary to minimise runway needed?

From here I wondered whether a smooth touchdown makes a big difference to the runway needed. Fairly obviously the extreme is true: a very smooth touchdown with a badly judged touchdown point will need lots of runway.

Administrator
Shoreham EGKA, United Kingdom

From a physics point of view, this must be true: At any landing you must transform potential energy (altitude) and kinetic energy (speed) into friction to come to a stop. So if you touch down firmly, you simply transform energy faster. The most “effective” way to do this is to crash, then you will need the least runway… the only difference between a crash and a firm, short landing is that the landing gear withstands the impact…

Low-hours pilot
EDVM Hildesheim, Germany

Peter wrote:

From here I wondered whether a smooth touchdown makes a big difference to the runway needed.

I think it comes down to the glide path. If you fly down the ILS and then flare for a smooth landing it will always be long as you are not allowed to aim for a virtual point before the touchdown zone. In a GA / VFR scenario you can do that and touch down super smooth at the desired point. But you will end up below the glide slope / PAPI on short final. One of the reasons my night landings probably turn out a little longer than day landings as 4 red does not feel good in complete darkness…

www.ing-golze.de
EDAZ

Sebastian_G wrote:

In a GA / VFR scenario you can do that and touch down super smooth at the desired point. But you will end up below the glide slope / PAPI on short final.

Why below? I find a steeper approach results in a shorter landing. Most of your excess speed (over stall speed) is converted to lift in the more significant flair, resulting in a slower touchdown speed.

EIWT Weston, Ireland

Peter wrote:

I wondered whether a smooth touchdown makes a big difference to the runway needed. Fairly obviously the extreme is true: a very smooth touchdown with a badly judged touchdown point will need lots of runway.

Indeed, hard landing minimises runway length, a short landing is done:
1/ By flying low threshold speed and steep approach, in theory, only Vat at 50ft matters but “attitude” comes into picture when burning energy due to acceleration & drag curve effects: you level off from -20deg pitch to +8deg you lose 30kts, level from -5deg pitch to +8deg you will barely lose 5kts

2/ By flare & float away from ground effect which is always followed by a hard touchdown
Hard touchdown “under ground effect” from inertia and flying into the ground (then stick it some breaks action or bounce due to excess of speed)
Hard touchdown “above ground effect” from high flare & float before stalling into the ground (it sticks by itself)

Flying 1/ is high likely to lead to 2/

Paris/Essex, France/UK, United Kingdom

It doesn’t have to be the case, but it is more difficult if you’re coming in steep and slow. Then using up the energy flaring requires you to judge the height pretty well. if flying along parallel with a very slow rate of decent for some time.

Hard landings often lead to bounces which definitely don’t minimise runway use.

MedEwok wrote:

From a physics point of view, this must be true: At any landing you must transform potential energy (altitude) and kinetic energy (speed) into friction to come to a stop. So if you touch down firmly, you simply transform energy faster. The most “effective” way to do this is to crash, then you will need the least runway… the only difference between a crash and a firm, short landing is that the landing gear withstands the impact…

I don’t fully agree with your application of physics ;-)

From the touchdown point to the stop we need to convert kinetic energy (no potential energy left) into heat. If we take out the “crash” option, where many parts of the aircraft contribute to this conversion and also look at SEP (w/o reversers, beta, etc.) this conversion is done by the brakes.
The speed at which the brakes can convert kinetic energy to heat is limited by two factors: The effectiveness of the brake system, so the maximum friction between the brake discs and the brake pad and the maximum friction between the wheel and the runway.
Let’s assume we have a sufficiently effective brake system that is defined by brake disk/pad friction not being a limiting factor – practically speaking you would be able to block the tires at any given point in time/speed/load on the wheels, In this case the wheel to runway friction is the only limiting factor.

Wheel to runway friction is (amongst many others) almost proportional to the normal force at the contact surface – therefore in good approximation proportional to the G-load at landing. That would be an indicator that a hard landing could actually reduce landing ground roll.
However (!) there is also a time component to that! You only reduce landing distance if you could maintain the more than 1g “force” on the wheels for some period of time. Hard landings, however, tend to have a quite hard initial shock but then actually also do the majority of the ground roll with just 1 g.

Therefore I would say:
– Obviously a firm landing should be aspired with “firm” being defined by bringing the full load of the aircraft on the wheels as quickly as possible after touchdown
– Beyond that I would not assume that a harder landing has much of a benefit due to the very short duration of this higher load. In addition even this theoretic advantage can only be realized with a perfect braking technique esp. when applying maximum brake power (just short of skidding) immediately after touchdown
- Without doing all of the numbers, I would therefore expect that the impact of a hard landing on required landing distance is by order of magnitude smaller than the distance you give away by common “mistakes” and inaccuracies, like, e.g. too high approach speed, missing the touchdown point, improper braking technique (i.e. applying full available brake power to late or skidding by applying too much brake power).

Germany

In the context of bush/STOL operations a hard landing isn’t necessary. FE Potts Guide to Bush Flying covered this point. Even if you watch videos of those mavericks in South America bringing Cessnas into 160m ish one way strips they don’t hammer them on. I once landed on a 173m strip for model airplanes and buttered it. Even by dropping all flaps in the flare which was my SOP in a tricycle gear going in tight, I don’t recall any hard landings being necessary.

In the context of jets or commercial aviation then I have no idea.

Buying, Selling, Flying
EISG, Ireland

It depends why the landing is “hard” for bounces (we’re talking here of “hard” landings well within the design parameters of the aircraft). A hard landing when there’s excess energy, and the aircraft is still at flying speed when it touches down is quite likely to end up in a bounce, as there’s lots of energy being stored in the landing gear system but not much weight on the wheels (because the aircraft is still flying). This is particularly true in a tailwheel aircraft, where a fast landing will mean the tail is still up and the tail can descend on a firm touchdown, increasing the angle of attack.

A firm landing where the plane has got done flying is much less likely to bounce (particularly in a tailwheel aircraft). A firm landing in the Auster for instance where the mains plant firmly after the tailwheel has touched down (tailwheel first landing implies a “full stall landing”) will reduce the angle of attack and plant the mains, and not bounce, and allow for immediate application of the wheel brakes.

Last Edited by alioth at 19 Jan 10:16
Andreas IOM

At least in commercial aviation, “hard landing” is a defined term from the maintenance manual which requires an inspection. The term you’re probably looking for is a “positive landing”, or maybe as in this Airbus presentation you could call it a high vertical G landing local copy if it’s harder than what you intended.

Given that you cannot nail the flare perfectly 100 % of the time, the question is if you should rather aim to the “soft” side of the distribution of G loads or the “hard” side. Evidence suggests that it’s safer on the “hard” side if you also adhere strictly to touching down in your intended touch down zone, and go around if not.

Last Edited by Rwy20 at 19 Jan 10:21
42 Posts
Sign in to add your message

Back to Top