Landing distance vs. approach angle

[Cloud surfer]

Anyone want to take a crack at this one?


What is the effect on landing distance if the approach angle is steepened, approach-speed and configuration kept the same?


Or, to use an actual case. If the CAA "lifts" the approach angle indicated by the VASI/PAPI from 3.3 to 4.5 degrees, and you fly the approach (maintaining the VASI/PAPI) as you always have done, and flare correctly and at the correct Vth, how will this affect your landing distance if you brake/reverse as you've always done?

And if there's an answer, does anyone have any "official" material backing this up?

[stillin1]

can't back it up but
my logic suggests that as you will need to start to flare slightly higher on the 4.5 deg app to arrest the rate of descent ~ the time / distance to the subsequent touchdown (at the correct speed) must increase as your flare will take a little longer. Therefore the landing distance will increase too as you will actually touch down a very little further down the runway. The distance to stop once on the deck should remain the same.

If you eyeball it to be aiming a little low on the app just before touchdown you can still land on the numbers and it all stays the same.

The manly way of course is sod the flare altogether and dump 10 kts into the runway on landing :uhoh:

[Crossunder]

These are (presumably) Steep approaches, with a screen height of 35ft instead of the normal 50ft. As long as you actually cross threshold at 35ft and Vref, the steeper approach angle will give the shorter landing distance. However; if the approach is flown below the PAPI/PLASI all the way down, the touchdown point would be closer to the threshold, and thus you'd have more runway left when the a/c has slowed to taxi speed.

An approach flown below PAPIPLASI would reduce obstacle clearance and increase the risk of CFIT in case of an unexpected downdraught. At night, the PAPI/PLASI will be almost the sole means of vertical guidance, especially since STOL ports (which I assume this discussion is about?) have a narrower runway and more uneven tarrain surrounding them, thus increasing the chance of optical illusions. Once below PLASI (flashing red)/PAPI(all red), you no longer have a good source of vertical guidance, as the PLASI/PAPI will not be giving any other information than "too low"; not much too low, or "slightly" too low.
Also; if your AA is flatter than the PLASI angle, and you are below PLASI, you could suddenly go from flashing red to flashing white in a split second when passing threshold... This would most certainly increase your lsg.dist.

Our Company has put a lot of focus on maintaining the PLASI/PAPI until passing threshold, but this discussion always pops up come winter and icy runways. Should you risk CFIT prior to touchdown, or hope that you'll be able to execute a perfect flare/touchdown and not slide off the runway?

Not an easy one this - perhaps someone at the tech- or engineer forum could shed some light on the subject?

Cheers!

[mad_jock]

I think i will go for the landing distance will be exactly the same.

Logic if you land in the same configuration at exactly the same Vref the aircraft will have exactly the same energy whatever the approach gradient is.

Once the wheels are on the deck you have exactly the same amount of drag acting on the aircraft so it will slow the same amount and therefore use the same amount of tarmac.

The only difference between a steep approach and normal is the amount of power or spoiler required to maintain profile.

MJ

[Mad (Flt) Scientist]

That may be somewhat true for the ground portion of the landing, but the "air distance" part from threshold to touchdown will be rather different if the approach angle differs, with shallower angles corresponding to longer air distances. "Landing distance" comprises both air and ground portions.

(I say "somewhat" because chances are the details of the flare will be different, so the actual touchdown speed may be different by some small amount)

[mad_jock]

True the you will get more drag in the flare due to the extra elevator drag pitching the nose up.

Fair point about the air distance

35/4.5*60= 467ft

50/3.3*60 = 909ft

So the ground distance is the (near as) same and the air distance is nearly half reduced

MJ

[FlyingForFun]

I would have thought that the landing distance would be shorter for a steeper approach. The flare will, as has been said, need to be quite a lot more pronounced. This will result in more speed being washed off during the flare, and therefore less float and an earlier touchdown. Depending on pilot technique, the touchdown speed should not change... but the way many pilots of tricycle-geared aircraft fly, you might find the touchdown speed, in practice, is lower (because it is too high from a "normal" approach) and therefore the distance required is less, too.

Not sure if there is a parallel with helicoptors here, because I know very little about how helicoptors approach and land. But I would imagine, if we take things to extremes, that helicoptors require zero distance to land if the approach is vertical (i.e. extremely steep) - and this might back up my argument. But I'm not sure.... any rotary pilots care to help me out?

FFF
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[4Screwaircrew]

When Stansted work on the runway over the winter they put in temporary PAPIs with a steeper slope than when on the full length. The reason being that if flown properly this will give a shorter landing distance, landing is never a problem but finding the runway off the 3 mile SRA can be a pain.

[Stick Flying]

Jock of the lunatic variety,
as landing distance required has an airborne portion
Therefore distance is reduced.

To all others:-

Only problem is if you try and maintain a steeper approach, the margins required to either 1)not overshoot or 2)flare at the right time, are compromised, hence the reccommended slope is 3 deg and rightly so.

[mad_jock]

Yep i will go with that.

Does the maths look right above?

MJ

[Stick Flying]

Maths almost right. The 60 rule not 100% accurate. I get distances of 867 and 445.

[john_tullamarine]

Conventional landing analysis considers

(a) approach (air) distance
(b) flare distance
(c) ground distance

with a bit of jiggery-pokery to join them all together.

(a) steeper approach might reduce the steady air distance, but increases the workload and judgement required for the flare. In addition, for normal jet speeds, the ROD will increase ... and we really are getting up to the point where a mishandled flare will cause a landing impact exceeding the drop test loading.

(b) the steeper the approach, the more skill is required for the flare - unless a carrier landing be the call ... see drop test loading. While I have no experience of steep approach angles, I would guess that the flare skill required is moderately high to achieve any sort of reliable repeatability ?

(c) for the same presumed touchdown speed, there ought not to be any sensible difference in ground roll for maximum energy stopping conditions .. and, if the speed spread is significant, the AFM data is going to suffer and we would have to wonder what the point of the exercise was ... ?

[alf5071h]

This is a copy of my post in Flight Testing on a similar subject.

In theory, a steep approach will provide a shorter landing distance, but this is not necessarily the same as a duck under the glideslope manoeuvre. Both aspects depend on defining and fixing some of the variables. e.g. landing distance required is measured from the threshold – from threshold crossing height (normally at 50 ft).

For a duck under approach, with the same aircraft, landing speed, flare technique, flare height, etc, the aircraft arrives at the ‘threshold height’ before the actual threshold (defining the landing distance required), this enables a touch down near the end of the runway. This apparently reduces the landing distance – feel good factor, but in certification terms, there is no reduction. However, there is the increased risk associated with destabilising the approach and eating into the obstacle clearance margins, which is a less safe operation.

The touch down geometry resulting from a duck under normally gives a flatter arrival that for some aircraft / pilots appears to make landing easier. However, whilst the flatter arrival is perceived to give a more accurate touchdown the spread of error in touchdown position increases at shallow angles as described by BOAC. Similar arguments can be made for airspeed and the dissipation of energy but these involve more variables.

For a true steep approach (>4.5 deg) where the glideslope angle is maintained until flare, and again with the constants as above, the landing distance is slightly shorter due to the GS origin (aiming point) being closer to the runway end and there will be less spread in touch down position due to any error in speed / technique.

The certification of the BAe146 / Avro RJ aircraft on steep approaches resulted in a reduction in the actual landing distance required (AFM performance amendment). I recall that the benefit ranges some 80 -130 meters depending on the aircraft variant. These values resulted from the combination of geometry, aircraft capability, and flight test; the spread of values were due to test technique, and that the larger aircraft have a restricted GS angle and wind limits.


* The geometric effect in part follows the description above, but the dominant term was the reduction in threshold crossing height from 50 to 35 ft, an even closer aiming point than for just the GS increase.
* The aircraft capability also relates to the reduced threshold height, the 146 wing lift and pitch control characteristics enabled the flare to be retained at 35 ft, avoiding any longer distance associated with an early flare. Alternatively, the Saab 2000 can fly a steep approach but due to its steep-approach flare characteristics (circa 60 ft) no landing distance benefit is allowed and there may even be a penalty.
* The other advantage that the 146 has is that during the certification of steep approaches (first jet ever?) many of the landings were measured for performance calibration. This followed from BAe’s use of the ’steep approach’ method of assessing normal landing performance during certification. Thus, there was considerable evidence as to the aircraft’s actual performance opposed to its projected performance. I also recall that the CAA did an in-service check of 146 landing operations at LCY in a similar way as they did for the DHC-7, a report is published somewhere.


IMHO approaches should be flown in accordance with manufactures advice, within the certification rules and assumptions, and with the consistency and stability promoted by safety initiatives, all of which add up to the safest operation - do not duck under. See also the discussion in thread ‘VRef speeds and half the HW comp.’

[Rivet gun]

One of the reasons the BAe 146 gets a reduced Landing Distance Required for steep approaches is because there is less statistical scatter of touch down point, and therefore of achieved landing distance.

At LCY however there are touch down cut off lights, i.e if the wheels are not on the ground by the lights you go around. Threfore pilots make a big effort to touch down before the lights.

When evaluating the stastistics of touch down point this may account for some of the reduced scatter at the upwind tail of the distribution, i.e. the reduced scatter may not be due just to the steep approach.

[safetypee]

Rivet gun - reduced scatter due to cut off; not necessarily so. The 146 landing performance was not predicated on the performance at LCY, but was determined from flight-testing on steep approaches elsewhere.

Extracts from a BAe safety presentation used some of the data in the CAA report and indicates that 5% of the touchdowns at LCY were beyond the fixed distance markings (only just beyond). The overall distribution was skewed with the median about 800 ft in from the threshold.
The lights inset into the runway are fixed markers to aid the crew; however, they should be used as you suggest.

JT your points reflect commonly held views, which in my experience are not necessarily true. Increase in judgement / workload / skill; yes, there is less margin for error in commencing the flare, but the mechanics of flaring and judging the touchdown are basically the same as for any landing; height rate vs height. Pilots quickly adapt (3 landings) and retain the ‘skill’ in a similar manner to remembering the change in approach and flare during an engine out / glide forced landing.

At some time during the CAA trials (by chance), the FAA filmed landings at LCY as part of a research project on aircraft gear loading; they held the view that steep approaches would increase the average load, presumably due to a spread of pilot judgement. To their surprise, there was no such correlation, nor do I recall any 146 suffering gear problems, but one wouldn’t with such a robust design would they.

The skill in the flight testing is to flare as late as possible and stay within the test conditions of not more than 6 ft/sec (and not less, which performance engineer requires).