New Instructor page: "Touchdown"

[Britjet]

Hi Charles,

It is a bit of a minefield potentially, but like most things in aviation, it can generally be broken down into common sense.
Firstly, there is (as far as I know) no technically-binding definition of "Limiting runway". I think it is reasonable to consider the phrase as one which describes the cut-off point in the calculation.

The flight-planning stage and the actual environment at destination are two separate processes. (Weather minima are dealt with in a similar way). The flight-planning stage is a complex process which includes the regulatory margins and percentage increments to weather forecasts etc to arrive at a reasonable probability that the runway will be used. This is not a small calculation. My performance manual for the 747 has an 8-stage process which follows a flow-chart system to decide whether a runway is "usable" for flight planning purposes. The cut-off would be if it was "limiting".
This is not a minor calculation and is not normally undertaken by the pilot. The flight-planning computer does it.

When you actually get to the airport you check your maximum landing weight on the tables (which have different margins built into them depending on normal or non-normal conditions) and if you are above the weight then you can't land. You use the actual weather conditions, even on short final. So for example, you might calculate that you can accept a 6-kt tailwind on final. If the reported tailwind is 7-kts, you should go around, although it is possible that some pilots might suffer short-term deafness in such a situation..:-)

Peter.

[tango4]

Many thanks for your explanations and patience with me on that one Peter.
So if I follow you correctly, in this example, you calculated that more 6kt tail wind implies go around.
But the simple fact that the result of your calculations is that you cannot accept more than 6kt tailwind means this runway would be deemed limiting (according to BA definition).

I think I'm finally seeing things a bit clearer thanks to you !

And as I said above, once you go into details, flight planning and regulations is indeed one of the most obscure (and poorly documented) topics for simmers. Although in the recent years programs like PFPX and TOPCAT really did a lot to introduce much more advanced concepts to the simming world.

Charles

[Hardy Heinlin]

If I'm not wrong, I would put it like this:


There are two kinds of limitations:


(1) Limitations of the aircraft (stall speed, max tire speed)

(2) Limitations of the runway (length, weather)


If (1) are reached before (2), then (1) are the primary limitations ("aircraft limited").

If (2) are reached before (1), then (2) are the primary limitations ("runway limited").

Whichever limitation is reached first, that's the primary limitation.


Another picture:

(1) You have nerves of a certain strength and you want to see a horror movie

(2) The movie is for adults only

If your age lies below the limit, the situation is "cinema limited". Otherwise it's "nerves limited".

Whichever limitation is reached first, that's the primary limitation.



Cheers,

|-|ardy

[Britjet]

Exactly Hardy. (I think.)
Peter

[tango4]

Thanks to both of you !

Hardy, did you talk to my wife before making that post? Because i happen to be complete wuss when it comes to those movies.
And by the way, you can also be "nerves limited" during landing. Not sure it is mentioned in the performance manuals though...

Charles

[Kieber]

Thank you Peter for attending and the correct presentation of this complex talking points.

And for Charles just some additional considerations and simple examples:
(If I am wrong, anybody in this forum is invited to correct me)

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 ?
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 ?

An approximate order of events if you like to fly to an airport with only one short runway.
According to the airport facilities and the aircraft status you decide the type of approach. (precision- or non precision-appr.). You calculate your allowed landing weight according to the rwy length, rwy slope, rwy contamination (braking action) and for the expected weather conditions at time of arrival (TAF) like pressure altitude, temperature and wind (especially maximum crosswind component),. Forgive me if I forgot anything. Assumed this implies that the allowed landing weight for your flight will be less than the maximum landing weight. Now you have to calculate the fuel in such a manner (fuel planning, another story) that you have to arrive your destination with your allowed landing weight, which is in this case (calculated) less than the maximum landing weight. (If the landing weight is greater than the allowed landing weight you have to fly some holdings to reduce the weight to the allowed landing weight).
When approaching your destination, you check the actual weather (initially from the ATIS, if no ATIS from tower), and prepare your approach and landing for the existing runway condition (contamination), and according to the actual weather data like pressure altitude (altitude, QNH), temperature and actual wind conditions.  Now you determine your speed bug setting and the auto brake setting. During your approach you can request wind readings from the tower (wind on ground).

Regarding your wind correction problem:

As Peter said, different airlines may have different interpretations and I assume, my explanations regarding the speed bug settings for the wind corrections are airline specific.

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

I think you were a little bit confused about my saying "In wind conditions of more than 10 kts we added a wind correction." I just want to say that in wind speeds between calm and 10 kts you always have to set the speed bug to the absolute minimum of Vref + 5, because if the wind is exactly head on with 10 kts, ½ the headwind component is not more than 5 kts.

Now to the crux "1/2 the steady wind if not runway limited".
It just means for easier computation* you can take the full wind speed for the correction (but maximum Vref +20). 
*You don't need to calculate the headwind component.
If the available landing runway allows a landing weight below MLW, you are runway limited and therefore you must use Vref + ½  the headwind component.
An example: Wind speed is 26 kts, headwind component is 16 kts. If runway limited, the landing runway is exactly for your landing weight, use Vref +8. (1/2 the headwind component).
If the available landing runway allows the MLW or above you are structural limited (by MLW!) and therefore you have additional landing run available* and so you can use Vref + ½ the wind speed). In the above example Vref + 13.

Finally examples for the speed bug settings: (remember it maybe airline specific)

Runway direction 360°. Reported wind 030°/10 kts
cross wind = 5 kts, head wind = 8 kts, ½ the headwind = 4 kts
Speed bug set to Vref + 5

Same runway. Reported wind 030°/25 kts
X-wind = 12,5 kts, headwind 20 kts, ½ the headwind = 10 kts
speed bug set to Vref + 10

Same runway. Reported wind 030°/30 kts, gusts up to 45 kts
X-wind = 15 kts, headwind 24 kts, ½ the headwind + full gusts (from 30 to 45 =15) = 12 + 15 = 27 kts
speed bug set to Vref + 20

*What about braking ? Do you assume MAX autobrake ?

It is estimated that manual braking techniques frequently involve a four-to-five second delay between main gear touchdown and brake pedal application even when actual conditions reflect the need for a more rapid initiation of braking. This delayed braking can result in the loss of 800 to 1000 feet of runway. Dirctional control requirements for crosswind conditions and low visibility may further increase the above delays as can the distraction arising from a malfunctioning reverser system.
For these reasons, it is strongly recommended that the autobrake system be used in preference to manual braking.

The following is offered as selection criteria:
-Max. Auto: this should be used when a minimum stop distance is required. However, the deceleration rate is less than that produced by full manual braking.
-3 or 4: These settings must be used for wet or slippery runways or when landing rollout distance is limited.
-2 or 1: These settings will provide a moderate deceleration effect suitable for all routine operation.

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.

Yes. (If you have only a little margin from allowed landing weight to the MLW maybe you catch the MLW with max breaking).

HTH
Walter

[tango4]

Thanks for taking the time to come up with this explanation.
It's VERY clear and well explained. I'll definitely save it for further reference because it really helps me understand what is calculated when and for which purpose.

Such a small initial question...which ended up with so much information !

As always, I am grateful for this forum and the knowledge and patience of some users. Given my slow understanding, I guess I would have been yelled at in some other places.

All the best.

Charles

[jtsjc1]

Thank you Peter and Walter. Having you, Hardy and others here on the forums to explain RW procedures and technical info on the 744 its like our own flight school.  I wish I could buy you all drinks! If you're ever visiting EWR I'm about a half hour away!

[Britjet]

Hi Joe.
A pleasure, and I must say that this forum keeps what is left of my grey matter churning.
Thanks for the invitation!
Peter

[Kieber]

Hi Peter, I agree and I still have your words in my ears:

PSX will bring it all back to you Walter - it will be as though you never left!
Peter

You're bang on!...but maybe there's a fly in the ointment... For me it is like to go through a sound barrier (existing of my family) coming up here... Maybe my afterburner needs a maintenance check.....

Hi Joe, As well thanks for the invitation!

Cheers
Walter 

[CarlBB]

Hi All,

On a recent approach everything was bang on the numbers ... except pitch and therefore the touchdown rate was 750 fpm. Ouch!

My question is - as I did not damage anything in PSX, what would actually happen i.e. would the aircraft require a check to make sure ?

I of course am buying the rest of the crew any such beverage they may require. Any maybe new clothing .....

Thanks

Carl

[Jeroen Hoppenbrouwers]

I believe that an aircraft needs to demonstrate that it can safely complete an autoland without the flare kicking in, which leads to a touchdown at 700-800 fpm. It may get damaged doing so, therefore a hard landing inspection is required.


Hoppie

[Will]

As Hoppie says, hard landing inspections are required after hard landings.

My question is: how much subjectivity goes into determining whether a landing was "hard" or not? There are enough devices on the aircraft such that a landing over a certain G threshold could trigger an automatic report to the engineers, but I don't know if that's implemented or not. Any ideas?

[Hardy Heinlin]

700 fpm on initial contact. Assume the time from there until full strut compression is 0.3 seconds.

700 feet/min = 3.6 meters/sec

3.6 meters/sec / 0.3 sec = 12 m/sec/sec

1 g = 9.8 m/sec/sec

12 / 9.8 = 1.22

So I guess 700 fpm will produce approximately 1.2 to 1.3 g.

What g value requires an inspection? 1.8 g?


|-|ardy

[Will]

Good question, Hardy. Transport category aircraft are certified to 2.5 G, so perhaps any touchdown that exceeds 2.5 G requires an inspection. But, it's entirely possible that the landing gear have a different reference tolerance than the wings. And of course, I still wonder if that is something that is reported automatically or left to the flight crew to report if they felt the landing was particularly hard.

[John H Watson]

It's actually the pilot who decides if a heavy landing has taken place. Then the g's are looked at.
It's either 1.7 or 1.8g, depending on the sampling rate of the recorder.
If the aircraft has more than 2 degrees roll, the recorded g's can be significantly less, but an inspection is still required.

[Jeroen Hoppenbrouwers]

I understood that Airbus aircraft automagically print out a landing report on the flight deck printer when the system thinks it may require an inspection.

The modern wireless QARs also offer the same function. We had once a customer making what the pilots determined to be a hard landing (bad weather) which usually means the aircraft is out for a few hours for the inspection. While still taxiing, the techs looked at the server and saw the flight data dump coming through. The pax were not yet deplaned when the green light came that no further inspection was required. That event paid for the whole fleet's installation of the QAR...


Hoppie

[John H Watson]

Does the Airbus software compute roll? Or if the nose hits first 

[Hardy Heinlin]

My calculation above with 0.3 sec deceleration time is probably too optimistic. It may be 0.2 sec or even shorter.

With 0.20 sec : 1.84 g

With 0.15 sec : 2.45 g

With 0.10 sec : 3.67 g

...

|-|ardy

[CarlBB]

Thank you for the replies.
Carl