New Instructor page: "Touchdown"

[Hardy Heinlin]

Then let us consider this a manual landing analysis only, and I'll replace the words "circa 200 fpm" in the text by "circa 350 fpm".

Any comments re crab and centerline offset tolerances?


|-|

[John H Watson]

Technically speaking, 450' beyond the G/S transmitter.

*******EDIT*********

Not to open a can of worms, but I am pretty sure the autoland touchdown point is actually about 2500' from the threshold.

[Hardy Heinlin]

Technically speaking, 450' beyond the G/S transmitter.

What is the question to this answer? :-)

[emerydc8]

Hi Hardy,

Crab during landing was never something I gave much thought, except to the extent that I aimed to have no crab at touchdown. Actually, I was kind of surprised that my crosswind landing above even had that much crab, because my goal is always to kick it out in the flare and, in the process, touch down on the upwind gear a fraction of a second before the downwind gear touches.

That's not to say that I don't like the feature. It is interesting and presents a new parameter I never considered before.

Technically speaking, 450' beyond the G/S transmitter.

John, would you say that in most cases the GS transmitter is located 2,000' down the runway? I thought this was interesting:

Performance source definitions:

 Flight deck performance (FDP): ACARS performance. Preferred performance method.
 Takeoff and Landing report (TLR). Backup performance method if FDP is not available.
 QRH Performance in flight (PI).

[Hardy Heinlin]

John, would you say that in most cases the GS transmitter is located 2,000' down the runway?

Looking at the maps, I would say most G/S stations are located circa 1000 to 1500 ft from the threshold (along-centerline).

So ... technically speaking, autoland touchdown may be circa 1500 to 2000 ft from the threshold.


|-|ardy

[emerydc8]

I wonder if there's some fudge-factor built into the Atlas data. Or maybe there's a software setting to have it touch down 2500 from the threshold. In any case, this data had to be approved by the FAA.

[Hardy Heinlin]

Maybe it's FAA specific?

[emerydc8]

Maybe you're right. I just checked my company's manual. They switched to Aerodata too, not long ago. Looks like it assumes a 2500' touchdown point if you select autoland.

[cavaricooper]

I suggest you watch my video "landing". In there (somewhere) I actually talk the "30, 20, 10" rate as it should sound.
This assumes a rate of descent that is held constant all the way down to thirty feet. We used to teach that the flare starts at the T in thirTy. Any raising of the nose before that that will result is an extended landing distance, and anything after that is likely to result in a hard landing.

I have done a few landings recently in PSX and it is my opinion so far based on my own efforts that you should be getting a touchdown rate in the region of 300 to 400 fpm. I think 200 is a "floater". (Sorry Hardy).

I will post some more info on this after I get chance to experiment further.

Flare pitch should not go higher than 5 degrees ideally. With a little as 7.5 even with wings level you can get a tail-scrape. So much so that it requires a callout of "Attitude" or similar in some airlines.
If you want to do it properly don't try for "greasers" ;-)

Peter

Peter- you're a star! Today, I wake to find I'm not as inept as I believed (Well.......). Back to hauling toasters around the world!



Best- C

[tango4]

Hi again guys,
Quick questions for the pilots here. I'm looking for a clarification about the explanation that Walter (KIEBER) wrote about Approach speed correction:
In wind conditions of more than 10 kts we added a wind correction. The recommended wind correction was 1/2 the steady headwind component (1/2 the steady wind if not runway limited) plus all of the gust value, based on tower reported winds. The maximum wind correction should not exceed 20 kts

My question is: could someone explain the term "runway limited" in the landing context. I have trouble getting this notion right in my brain. If you plan to land on this runway, it means that when you made the computations, your landing distance is less than the runway length ?

I'm obviously missing something obvious but I can't figure out what !

Charles

[Britjet]

Different airlines may have different interpretations but generally it means that you can't land at maximum aircraft landing weight with a 10kt tailwind.
Peter.

[tango4]

Thanks a lot Peter !

Charles

[Britjet]

I should have said, of course "greater than a 10kt tailwind"...
Peter

[tango4]

Just a confirmation to make sure I understood. This means that this calculation is basically made once and for all (for a given aircraft type).
As we are talking about MLW, this means that before taking off, you know if your landing runway at destination is limiting or not. This is not a calculation made on the fly as it does not refer to any current conditions at the time of landing.
Am I understanding correctly ?

Charles

[Kieber]

Hi Charles,

just a try to explain your question regarding "runway limited".

If for a given runway length your calculations for a landing resulting in an allowed landing weight which is less than the airplanes maximum landing weight, you will be runway limited. This means you can't land with the airplanes maximum landing weight on this runway! (The short runway is reducing the airplanes maximum landing weight)

HTH
Walter

Reg. your last post: But of course, all calculations are done before flight and when the existing conditions are poorer than allowed, you have to divert.

Sorry I'm a little bit under deadline pressure (family!), maybe later more.....

[tango4]

Thank you both for your help.
I'm not completely done understanding this !
Your answers lead me to some other questions (when you have time, obviously, we all have a "real life" !)

So, to know if you are runway limited, you start a landing performance calculation using:
*the given runway length (obviously, this is a fixed value)
*you assume aircraft at MLW
*what about weather conditions ? Peter, you mentioned 10kt tailwind. But for the rest do you assume STD ISA, or forecasted WX ?
*What about braking ? Do you assume MAX autobrake ?

So, in order to declare that the runway IS limiting, it means that using max braking power, you WILL NOT be allowed to land on it at MLW.
Am i getting closer to understanding or is it completely different ?

Thanks again guys, I really appreciate your help.

Charles

[United744]

I don't have the manuals in front of me right now to check, but doesn't the FCOM or FCTM give any Boeing recommendations on maximum crab angle at touchdown? I know for the A32x family the maximum crab angle at touchdown is 5 degrees due to side-loading of the gear.

Generally I decrab aggressively, allowing for the fact the aircraft has a large mass and so the side-force is high even at relatively small angles.

There are numerous examples on YT of how NOT to do it. In some cases, no attempt is made to decrab! :-O

This is the correct way:

http://www.airplane-pictures.net/photo/30509/f-gisb-air-france-cargo-boeing-747-400f/

[Britjet]

Hi Charles,

Yes, I guess what I said was a tad confusing. Walter's explanation is the more correct. Mine was a generalisation that just took a guess at "worst case", which might be a consideration if you set off without knowing the conditions ahead of time. For example, if you set off to an airport where the only available runway was limiting with the worst case wind conditions, with the landing weight that you were expecting, then you might want to think more about possible eventualities..
The limiting factors are the conditions that you get on the day eg weather, wind component, runway state etc.
One factor of a limiting runway is that most operators require full reverse to be used on landing in this case.

As for braking - there is no requirement to adjust braking to maximum simply if runway is limited - remember that calculation of maximum landing weight is regulatory and includes "safety" margins, (typically 67% on a dry runway), depending on runway state and so on.
What you actually use on the day is up to you, but of course these braking levels should be calculated from the braking tables in the QRH.

HTH

Peter.

[tango4]

Hi again Peter,
I think I'm starting to understand things.
What is confusing for me is the fact that we are indeed mixing elements of planning with elements of real time in this discussion( which i imagine are pretty obvious for you as this is your job but can be pretty confusing in the sim world, especially when you are digging deeper in the subject !)

For example, I know that you can select the autobrake notch you want for the ACTUAL landing, depending on the live WX conditions and performance charts.
So on that specific point, my question would be:

You are still on the ground, planning your flight, and wish to know if your planned destination runway is limiting or not. To know if you CAN land on this runway, you must somehow get your planned landing distance at MLW. But this landing distance will of course vary with braking power you chose. So would this calculation (the one you make when you are still on the ground) be based on MAX MANUAL Braking, or some other reference ?

What confuses me also here is that the quote initially made by Walter was talking about the wind additive that you apply on final, meaning real time. So this means that at this moment, you KNOW whether this given runway is limiting or not. But at this point in flight, you normally have made for final computations using your actual landing weight and live wx conditions. The result of those is that you CAN or CANNOT land or this runway. And when both of you are trying to explain to me this notion of "limiting" runway, you mention the fact that you might divert if the runway is limiting: I understand that. But once you are on final, you have made the choice not to divert because in the ACTUAL conditions, you have computed that you CAN land. So why would you care anymore about the fact that it was possibly a limiting factor (during flight planning it sorts of tell you that you have to pay attention to the actual conditions at destination and be prepared to divert) and include this in the ACTUAL Vref additive ?

I'm terribly sorry, I just gave myself a terrible headache even writing this. So reading should be worse...

I'm just very slow on that one !

Charles

[tango4]

Perhaps I am looking at the problem the wrong way.
Would this definition be correct :

You can consider that the runway is NOT LIMITING if you can plan to legally land on this runway at MLW under any wind condition (up to a certain tailwind, after which you will have to use the opposite QFU).

Would that make sense ?