Q for a performance expert

[dragon]

G/A WAT

Can some smart guy point me to a good and credible explanation to this question;

Why do Boeing and Airbus employ engineers to calculate Go Around WAT, and why is Go Around WAT published in the respective AOM Performance sections?

Given that, and simply stated;

Takeoff data is calculated from 35 feet above the departure end of the runway, in the takeoff configuration, to meet either second or third stage climb with the critical engine inoperative.

Go around WAT is measured from the MAP [higher and earlier than T/O], in the landing configuration, to meet either second or third stage climb with the critical engine inoperative.

The genesis of this question came about with a person’s belief that if an aircraft could meet the T/O WAT, it could meet the G/A WAT. Of course this is incorrect.

Any takers with the REAL reason and a reference to copy and paste?..

Many thanks

dragon

[MAYDAY]

Not sure if there is more too it, but this is my understanding. It is quite possible that you would be in a situation where you are on approach to a runway which you would not be able to takeoff from due to WAT limitations. Go Around WAT data allows us to land on runways which we would otherwise not be able to use.

[Innuendo]

If I read your question correctly you are asking why, if you can take off from point A you cannot also G/A from the MAP for the same runway in the same conditions. Good question, from a performance point of view I can't see why not unless the terrain is very unusual. Could it be that the published G/A WAT limits are generic so that you can determine capability for a G/A at an airport other than the departure point?

Airbus also include in the A-340 overweight landing QRH, a chart for maximum weight for G/A for airport elevation and temperature

that should be observed if circumstances permit.

[B75/76]

I believe, simplistically, it has to do with the fact that our landing configuration and take-off config are different. Generally much more drag when landing and therefore the ability to fly away at a given weight is greatly reduced.

As an example, going into YYC on a hot day with the F28, with a given flap, one would be restricted by the ability to climb in an overshoot rather than the ability to land and stop on the runway. Less landing flap allowed for greater landing weight.

Not sure my ramblings make any sense. As with most discussions, I know what I mean. :-)

Cheers (

[B75/76]

Well, your question got me to thinking (as painful as that is!) and I felt my previous answer was lacking. So, you asked for it… ;-)

The weight we can take into a given field is limited by ‘demonstrated landing distance performance’, ‘approach climb performance’ and ‘landing climb performance’.

I understand your question to relate to approach and landing climb limits.

These are to ensure minimum climb gradient in certified approach and landing configuration for a go around at any point in the landing approach.

Approach climb gives a 2.1% climb gradient with approach flap, one engine inop, gear up, G/A thrust and spd not above 1.4 Vslg.

Landing climb gives 3.2% climb gradient with landing flaps, all engines, gear down,spd not less than 1.23 Vslg. T/off thrust reached 8 sec. After thrust levers advanced from idle to T/off position.

The more limiting of the two gives the climb limited landing weight. Approach climb is generally more limiting for 2 eng a/c and landing climb is generally more limiting for 4 eng a/c.

Limit weight accounts for airport temp, pressure altitude, engine bleed and icing, if conditions encountered during flight and landing temp less than 10 c.

The JAA also has a low visibility requirement for DH less than 200 ft.

As these considerations are much different than those for T/off, the weights for landing and T/off can be different for the same rwy.

Disregard the above rambling if I didn’t correctly understand the question.

Cheers (

[dragon]

Not sure I am following you MAYDAY.

[dragon]

”Could it be that the published G/A WAT limits are generic so that you can determine capability for a G/A at an airport other than the departure point?”

Perhaps. However, with airports as different as Saskatoon is to Castlegar is to the 26s in VR [GA heads out over the chuck at MSL] is to ZRH with its engine out [T/O] escape routes for specific runway/SID combinations, how does a generic platform physically satisfy all of these variables?

[dragon]

You guys have the question right, simply, why the need for MLW WAT data? Let me also say that I do not know the answer..

b75/76, are you trying to confuse me cause if you are, it doesn’t take much and you’ve already way surpassed my confusion threshold..

Here is a little more to chew on. AC 767 specific;

In order to retrieve takeoff data for your particular flight, you must input ALL of the following:

Aircraft series [200 v 200 ER and 300 v 300ER]

Engine manufacturer and series of similar hull [ie PW 4060 or GE CF6-8]

Station

Runway [but not SID]

Flap setting

OAT

ETOW

QNH

And then factor applicable penalties for:

Packs off

Engine anti ice

Tailwind

Contaminated Runway

Wet Runway

Intersection

Construction

MEL

Here is what the Boeing 767 AOM says about MLW for Altitude and Temperature.

MAX LANDING WEIGHT FOR ALTITUDE AND TEMPERATURE

This table shows maximum weight as a function of airport pressure altitude and temperature for which adequate performance exists to perform a go-around on one engine.

The ONLY information required to extract the required information is

Series [200 v 300]

Flap [25 or 30]

OAT

Pressure altitude

It is neither model nor power plant specific, not terrain specific, not runway specific, not even station specific.. simply 200 v 300, 25 or 30 Flap, OAT and PA? [penalties applicable].

SO, ”....for which adequate performance exists to perform a go-around on one engine."

To what? To where?.. still not getting it?..

[Moon The Loon]

It is neither model nor power plant specific...

Well, actually it is. If you have a proper 767 AOM, it is issued for that aircraft serial number together with specific limitations and performance data.

The only information one can glean from AOM performance data is what the aircraft can do given specific data (temperature, runway slope, aircraft weight, etc).

The numbers will not tell you if you will clear an obstacle. They will tell you what your speed and altitude will be at any given point in the climb.

That's where the Runway Analysis folks step in to fill the void. They then give us the maximum TOW for each runway (full length or intersection) from each airport subscribed to. These analyses are produced by many different companies. Jeppeson is one of the biggest and widely used. Each producer applies all kinds of other information to determine if Net Takeoff Flight Path can be met from any given runway.

They also include landing data which will give maximum landing weights for the same runway. These maximum weights take into account the approach climb limited weight and the landing climb limiting weight.

Or in other words, exactly what B75/76 said!

Not to mention the aircraft is in a much higher energy state on a go-around than a takeoff.

Is that headache getting any worse?

[dragon]

” Well, actually it is. If you have a proper 767 AOM, it is issued for that aircraft serial number together with specific limitations and performance data.”

I don’t know that we have ‘proper’ AOMs, I know we do have TC approved 767 AOMs. The limitations section is VERY explicit with respect to - serial number specific - data. The performance section is NON specific to the other extreme. Do you know of any major airline that provides a type specific AOM in the OBL?

” The numbers will not tell you if you will clear an obstacle. They will tell you what your speed and altitude will be at any given point in the climb.”

The numbers produced can not guarantee terrain separation even with strict adherence to the published lateral track of a specific missed approach for a particular runway. I disagree that ANY chart produced will tell you what your . " . . speed and altitude will be in the climb." How could that be? I agree with your comments about TAKEOFF WAT data. My question is for MAXIMUM LANDING WEIGHT FOR ALTITUDE AND TEMPERATURE contained in the AOM performance section ‘GO AROUND’. Again, the directive, from Boeing, states that;

“ . . . . for which adequate performance exists to perform a go-around on one engine.”

So, adequate performance for WHAT?

” They also include landing data which will give maximum landing weights for the same runway. These maximum weights take into account the approach climb limited weight and the landing climb limiting weight.

Are you telling me that an Air Carrier exists which provides, on the flightdeck OBL, performance charts for the MLW GA WAT specific to every runway that this specific equipment could possibly expect to visit? Obviously they do no, so if Boeing provides generic data for a GA on 26R in YVR [sea level] what information does that same generic data provide for runway 16 in Zurich.

No headache, . . . yet. I basically understand the concept but the initial question remains active, the answer a little murky. Still hoping for some clarity.

Howz it goin?

[Innuendo]

"how does a generic platform physically satisfy all of these variables?"

Good question, probably does'nt, it may be that as generic data it is very conservative and more restrictive than data tailored to specific airports such as Zurich where the info is more specificaly covered. Your header for the thread did say "Q for a performance expert" so perhaps I should not be spouting off here , interesting question though.

[dragon]

” Well, actually it is. If you have a proper 767 AOM, it is issued for that aircraft serial number together with specific limitations and performance data.”

I don’t know that we have ‘proper’ AOMs, I know we do have TC approved 767 AOMs. The limitations section is VERY explicit with respect to - serial number specific - data. The performance section is NON specific to the opposite extreme. Do you know of any major airline which provides a FIN specific AOM in the OBL?

” The numbers will not tell you if you will clear an obstacle. They will tell you what your speed and altitude will be at any given point in the climb.”

The numbers produced can not guarantee terrain separation even with strict adherence to the published lateral track of a specific missed approach for a particular runway. I disagree that the charts duplicated in the AOM will tell you what your . " . . speed and altitude will be in the climb." How could that be? I agree with your comments about TAKEOFF WAT data. My question is for MAXIMUM LANDING WEIGHT FOR ALTITUDE AND TEMPERATURE contained in the AOM performance section ‘GO AROUND’. Again, the directive, from Boeing, states that;

“ . . . . for which adequate performance exists to perform a go-around on one engine.”

So, adequate performance for WHAT?

” They also include landing data which will give maximum landing weights for the same runway. These maximum weights take into account the approach climb limited weight and the landing climb limiting weight."

Are you telling me that an Air Carrier exists which provides, within the flight deck OBL, performance charts for the MLW GA WAT specific to every runway that that specific equipment could possibly expect to visit? Obviously they do no, so if Boeing provides generic data for a GA on 26R in YVR [sea level and no obstacles] what information does that same generic data provide for runway 16 in Zurich with a lot of obstacles.

No headache, . . . yet. I basically understand the concept but the initial question remains active, the answer a little murky. Still hoping for some clarity.

Howz it goin?

[Vsplat]

Good afternoon. If I understand your question right, you are comparing landing WAT with obstacle performance. It's a bit of apples and oranges. Here's why.

WAT is a certification requirement. It is based on a gross climb gradient, without reference to any obstacle, and, by the way, not referenced to MAP or any specific height AGL. As the name implies, it is valid for a given weight, altitude and temperature, all of it in free air. It's an instantaneous value, not usable to predict the entire aircraft flight path, for example to the end of the missed approach.

This is entirely different from Net Takeoff Flight Path, which uses a degraded flight path value and is compared to real obstacle data.

As you know, WAT applies at several phases of flight, among them takeoff and landing. The two are linked for FAR 25 aircraft without fuel jettison, but that's another story. Landing WAT is measured in two ways: approach climb and landing climb.

Approach WAT climb is measured with flaps in the approach configuration, gear up and an engine inoperative. Landing WAT climb is measured in the full landing configuration, all engines operating. That is one of the reason for flaps to be reset immediately during a go-around, and for some aircraft, the delay of landing flap selection when approaching engine-out. Most aircraft do not consider engine-out climb in the full landing configuration. Nor do they generally consider the height loss during the transition from approach to missed approach phase (hence the Canadian Low Energy Go-Around Advisory material).

All this said, except for JAA operators, there is no requirement for NTFP-like analyses for the go-around. You are on your own in Canada and the US. Despite numerous debates between the FAA, TC and JAA, this issue remains unresolved.

Boeing and Airbus do provide net go-around flight path data for operators who wish to use it, or are required to do so for the JAA. As far as I know, it is not in use in North America, except for a couple of particular airports surrounded by hostile terrain, and in those cases, it is at the option of the operator.

References: I would suggest FAR 25 and the FAR 25 Flight Test Guide. These do a better job than the corresponding TC documents of setting out the requirements. Transport Canada has a guidance book, TP12772, Aeroplane Performance, however it deals with this part of things in only a basic sense. There is one chart in that book that compares the differences between certification and operating rules that you might find helpful.

I hope this helps a bit. If I missed part of your question, please clarify and I'll try to address it.

Cheers

Vs

[Guest neo]

Awesome tech info, V*. Thanks for posting.

neo

[Moon The Loon]

Do you know of any major airline which provides a FIN specific AOM in the OBL?

Major airline? Can't say. Canada 3000 did. The international charter carrier I'm working for now does. HMY does; Air Atlantic in the old days did. All serial number specific including the perf charts.

I disagree that the charts duplicated in the AOM will tell you what your . " . . speed and altitude will be in the climb."

"Speed & altitude at a given point in the climb" is another way of saying gross takeoff flight path. And yes, these numbers are calculatable given specific environmental data. The net takeoff flight path for a two engined transport category aircraft is reduced by 20% to give a minimum net take off gradient in the climb. It is this set of calculations which must be specific to each serial number/engine combination. These numbers are in the manuals in the OBL of aircraft I've flown.

So, adequate performance for WHAT?

Re-read b76/76's post. The answer is there. I can't explain it any better than he did.

Are you telling me that an Air Carrier exists which provides, within the flight deck OBL, performance charts for the MLW GA WAT specific to every runway that that specific equipment could possibly expect to visit? Obviously they do no, so if Boeing provides generic data for a GA on 26R in YVR [sea level and no obstacles] what information does that same generic data provide for runway 16 in Zurich with a lot of obstacles.

I don't know how much time you've spent on the flight deck of an airliner in active service. Yes, aircraft specific documents including charts specific to the engine are included in the OBL. I'll agree that perhaps they are not for a large carrier like AC or AA or the like. They will be found though in the flight operation's library and within the maintenance department.

The obstacle clearance data is in a separate document. I spoke of it in my earlier reply to your question. That is the runway analysis charts. They are not generally produced by the aircraft manufacturer, although at least Boeing will provide them on request for a very large amount of money. They are produced separately by companies that examine each and every runway subscribed to by the operator. And yes, Zurich could be included. Not only for one flap setting, but for all takeoff flap settings, from the end of a runway, or from an intersection, with or without a special procedure to avoid near in or distant obstacles deemed to impinge on the NTOFP. They will also provide power settings for full or reduced thrust, and lots of other things.

Several attempts have been made to answer your question. Don't know what else can be added.

[Moon The Loon]

Well said Vsplat.

BTW, where'd you get that handle? Sounds gross! Reminiscent of road kill!

[Vsplat]

This thread has gone a long way in a direction I won't comment on, however I should point out that the degradation from gross to net is not 20% as you suggest. It varies by the number of engines, but for a twin FAR 25 it's 0.8% below 400'AGL, 1.1% at and above 400' ( or higher depending on the start of enroute climb)

Cheers

Vs

[Vsplat]

I've tried to answer your original question below (NTFP vs WAT), however you've added a wrinkle here that I hopefully can speak to as well.

WAT data does not reference a runway, departure or anything relative to the ground. The WAT limits for every runway on an airfield should be identical for the takeoff configuration and WAT conditions. That is because WAT disregards runway length and obstacles.

However, WAT must be satisfied for every takeoff, as a condition of the C of A, so, yes, every airline in the world does this calculation for every takeoff. Most operators use a takeoff analysis which simply consolidates all of the limitations and provides a single, limited weight and associated speeds. In most cases, the limit is accompanied by a coding system that will indicate the reason for the limit. Sometimes it's WAT, most of the time something else.

Landing WAT is simply a measure of the aircraft' ability to keep flying. It does nothing to ensure obstacle clearance.

Before I leave the subject, I should comment on some cultural issues that have really added confusion. A major Canadian carrier has expanded their local definition of WAT to include things that are not normally associated with this value - such as obstacle clearance. They have also elected in some cases to alter the data (conservatively) to serve fleet commonality (instead of serial number specific data). All this has lead to some pilots thinking WAT means obstacle clearance, others not.

Does this clear or muddy the water? Not sure, please let me know if there is something I can address more clearly.

Cheers

Vs

[Moon The Loon]

When the gross gradient is reduced by 20% to achieve net, that leaves Net as .8 or 80% of gross. Simple math. You are right about the number of engines. Didn't want to make Dragon's head ache any worse.

The different segments all have their respective required gradient. But once the overall gross path has been calculated, that's when the 20% reduction or 80% of gross calculation is applied to come up with the Net Takoff Flight Path (NTOFP).

I'll disagree with some of your numbers though for transport category aircraft. Second segment NTOFP must equal 2.4% or greater for a two engined aircraft (Dash 8 and B767 as two examples); third and fourth are somewhat less as third is acceleration and fourth is enroute climb to 1500 or distant obstacle clearance height, whichever occurs later. Time limit on the engines' takeoff power setting can be a factor in some Special Procedures (KLAS).

I can't comment on lighter aircraft i.e. less than 12,500. Yeah, the thread has "moved along" but it is a very interesting topic, something which is not well understood.

Cheers back at you!

[Vsplat]

Well. I said I wouldn't get mired in this, but one more try.

The reduction is 0.8 percent gross to net. That is 0.008. Not 0.8.

As for your climb segment numbers, once again apples to oranges. You are quoting WAT (gross) certification values, not net takeoff flight path degradation values.

Your discussion of WAT takeoff climb values can be found at FAR 25.111 and 25.121. I suggest you look further, at 25.115 and 25.123 for the degradations.

For your consideration.

Cheers

Vs

[Vsplat]

Well, I said I wouldn't get mired in this, but one more try.

The reduction is 0.8 percent gross to net. That is 0.8/100 or 0.008. Not 0.8.

As for your climb segment numbers, once again apples to oranges. You are quoting WAT (gross) certification values, not net takeoff flight path degradation values.

Your discussion of WAT takeoff climb values can be found at FAR 25.111 and 25.121. I suggest you look further, at 25.115 and 25.123 for the degradations.

For your consideration.

Cheers

Vs

[dragon]

Please do not feel bad or even that you are different from a lot innuendo, me included. As Moon the Loon so aptly points out down the thread, a somewhat misunderstood collection of data issues.

During my last sims, the question arose. Five of us in the discussion, some smart guys too.. Final count;

Four different answers.

One ‘Deer in the headlights’

Have a great night.

dragon

[dragon]

B 75/76

I think you did understand and thanks for the time you took. I think I was initially thrown off a little by some of your terminology. Quite truthfully, I now realize that perhaps a better understanding of this issue is in my best interest.

I will copy what you have written and along with the contributions of the Loon and Vsplat, go do some research on my own. Sounds like a very interesting read.

I may be back with a few more questions.

Thanks again

cheers

[dragon]

Vsplat,

You have a sound knowledge of this issue, do you or have you instructed?

Thank you as well for taking the time. Very well presented and yes, you understood my question clearly. Thank you as well for the reference. As I stated above, I will take all the information posted here, as well, dig deeper into the FAR as you’ve suggested [i did start there but obviously have not gone deep enough].

If, after all that I am still unclear on some information, I will be sure and get back to you.

Kind regards

dragon

[dragon]

Yes, it was very interesting.

Been watching things unfold there in your front yard. Let me join others in wishing you and your family both safety and good fortune.

Our thoughts and prayers are with you and all those facing nature's ways.

dragon and family