B737 TakeOff Analysis Question:ASDA/ASDR

[Cap Loko]

Hello everyone,

On an average day, we use the T.O. analysis and we are, lets say either climb or field limited (as usual in our daily operations):

1-Is it correct that we are not balanced in these cases, and ASDA is significantly greater than ASDR?
2-Using the takeoff analysis in general, means that takeoffs are usually not balanced?

Excuse me for asking such a basic question but the cappies I fly with cannot answer these questions.

[rhovsquared]

Well, if your TODA is greater than your TORA (after rotation you're over a clear way) then you're unbalanced, so if the TO analysis artificially produced a clearway in order to unbalance a balanced field with the coresponding weight reductions, in order to produce that unbalanced field then you have and unbalanced field. In either case your ASDA can never be less than you're ASDR. as well you're TODA cannot be less than TODR. It would be helpful if you can provide a rundown as to how you do this analysis and what curves or tables you're airline requires you to have. but I assume most of you're operations are not BF... not sure if this is what you want but i tried :)
edited to add: if ASDA is much greater than ASDR then you can unbalance the field in order to produce a clearway i.e add some of the ASDA to the TODA, but it may require a weight reduction to ensure that the TODR = TODA

[mutt]

Its sad that your company didnt supply the answers during training!! :(

In order to answer your question, you need to ask the provider of your takeoff data. Boeing programs allow 5-7 options for selecting the type of calculation that you want to do, some operators will use Balanced V1 whilst other Optimum, Minimum or Maximum.

Mutt

[mutt]

rhovsquared

but I assume most of you're operations are not BF

Dont assume, we plan all our takeoff performance on balanced field :)

Mutt

[Cap Loko]

Thanks a lot for your replies and the patience to answer these basic questions,

Here is a rundown of how we do..
(figures are arbitrary as I do not have docs at hand)
given: Actual G.W.:71t / OAT 18°C / airco on
1-We have the option of fixed derate from 26k to 24k. So, usually we start on the 24k t.o. analysis page.
2-First look at the OAT and determine MTOW and if we are climb or field limited. So far I haven't seen any other of the 5 limitations in our operations. In this example I will use climb limited.
3-Then determine 'airco on' penalty + climb limited, usually 2000 kgs
4-71t+2t=73t will give us the assumed temp. Let's say 32°C
5-Then check V speeds consistend with actual weight: V1=143 Vr=146 V2=152. Crosscheck with FMC calculated speeds and confirm.

Given this scenario, we only know that we are climb limited but we do not know our ASDR for example except that it is less then ASDA of course.
We do not 'play'with distances to add to ASDA. We do however, reduce weight only if the standard rate of 26k will not give use the required performance.

My reason for the questions are the 'what if'scenarios; what if we have to abort at V1, what if we go n-1 at V1 and continue etc..Just like to know what i'm doing and more important, what the aircraft can and will do.
Thanks!

[mutt]

So far I haven't seen any other of the 5 limitations in our operations You wont see them as they are part of the input parameters, not the output.

Crosscheck with FMC calculated speeds and confirm FMC can only calculate Balanced Field V-speeds, so if you are using the FMC Vspeeds without corrections, the takeoff weight must be based on balanced field.

The logic of the Boeing chart will show the most limiting value in the zero wind column, the difference between the most limiting and 2nd most limiting could be as low as 10 kgs. You therefore have no way of knowing how much runway length is remaining. Its worth remembering that if you are using Assumed Temperatures for thrust/speed settings, the weights that you see on the chart are for Actual Temperatures, due to differences in the Thrust, TAS/IAS you wont be as limited as you believe.

Look at some of the other threads regarding the "what if" scenarios.

Mutt

[Cap Loko]

Mutt,

We always use FMC calculated speeds. Sometimes, these speeds differ a little from the t.o. analysis speeds by a few knots but according to the captains thats ok.:confused:

[mutt]

If there is a difference between your calculated V-speed produced with the takeoff weight and the FMS, it basically means that you are not using a basic balanced field. Looks like you really have to ask the producer of your takeoff data for the inputs....

Just remember, if there is a difference between the charted V-speed and the FMS, use the charted V-speed.

Mutt.

[Cap Loko]

Thanks again for your reply,

I will ask the performance engineers in our company (if I can find them...) and will post the answer here.
We do, however, use FMC calculated speeds even if there's a little difference with the charts. This puzzles me to say the least......

cheers

[mutt]

use FMC calculated speeds even if there's a little difference with the charts

Is that legal?? IMHO due to rounding errors a difference of 1kt is acceptable, anything else and you should use the same parameters that were used to calculate the weight, therefore you must use the charted values!


Mutt

[rhovsquared]

For any given set of limiting conditions, based on the data used by the airline and applicable regulations based on certification and airworthiness minima- there exist only on factor that can be controlled by the by the performance planners that is weight; so field performance (ASDA v.s ASDR) or obstacle clearance-acceptable performance AEO and OEI from the screen through the net flight path at the applicable airport's RWY's. So, by Regulating the Takeoff
Weight, for applicable limiting factors, to an almost unlimited variety of possible circumstances (from flight test data); the data held by a particular operator for a particular type depends upon 1. the certification, 2, the certification's authority's requirements, specific airline data that the may have at hand due to special operational considerations etc..
Types of limiting conditions/ factors may include the following 1. field length-ASDA, Obstacles, MEL's, Configurations (flap setting for TO), Power settings, Bleeds and Packs, MLW at next field, Rwy conditions, winds, OAT, operation's long haul or short haul, personal minima and so forth. So, for a huge number of limiting possibiliies a maximum takeoff weight is scheduled for that field and(whatever schedules and data exist.

Now your side is to see that your actual take off weight v.s chosen thrust settins (derates, assumed temperature for reduced thrust) and configuration 9flap, weather, winds, OAT, (CoG for stab setting), is within in the limits of the one dependent variable that you are truly responsible for is speed (now my above post I wanted to avoid saying Vee :\ ) So you set these speed based on whatever your data RTOW's are calculated and based upon. your TO analysis, as mutt has said, seems to be a gross error check for the FMC, becuse for the most part you cannot control weight and definitly not RWY conditions. Now, if your data is BF then so be it, if you have an approved choice of V1 speed if there's a big cross wind don't get too close to V1Mcg if you can help it (the ADM part). To find out for sure what data is being used and what assumptions are made and if your V speeds are BF or what ever, well ask the performance specialist how they calculate your RTOW's-I hope i dididn't make anything up:ouch: :8 :} :} :}

[Old Smokey]

Is that legal?? IMHO due to rounding errors a difference of 1kt is acceptable, anything else and you should use the same parameters that were used to calculate the weight, therefore you must use the charted values!
Mutt

Music to my ears! Would you believe that there is a certain major airline which advocates using FMC speeds if they fall within 3 knots of the Airport Analysis?

Play it again Mutt, play it again :ok:

Regards,

Old Smokey

[Cap Loko]

To better understand the FMC calculated speeds and their meaning, I have been looking in the Boeing(Vol.1) company manuals and docs to learn all the inputs of FMC calculated V speeds to no avail. Can someone point me in the right direction?


@Mutt,

I'm sorry to say but some captains I fly with, easily allow differences 2-3 knots. I feel it isn't right. Knowledge is power......you see my point.


@rhovsquared, thanks

[mutt]

Basically the FMC uses:

Weight
Thrust Rating
Flap Setting
Pack Conf
Slope (single)
Wind


It actually doesn’t care about the runway length, obstacles, climb limits, brake energy. These are accounted for in the specific runway takeoff weight calculation.

The problem with using the FMS speeds that are 2-3 kts different from charted value will only become apparent when it is too late~! Imagine yourself in the court of inquiry trying to explain why you spent 5 minutes at pushback calculating the limiting weight and speeds from charted values AND THEN ignored them!

I would strongly suggest that if there is a difference, that you overwrite the FMS speeds.

Mutt

[Cap Loko]

Thanks a lot mutt,

Makes a lot more sense to me now and see why there are differences between the T.O. Analysis and the FMC speeds.
I will discuss it in my company.