Unbalanced Field Length Performance

[Red Mud]

As the operator of an FMS performance database equipped aircraft I generally use it to determine all V speeds and take-off/land distances. If the numbers are marginal I will use the AFM performance charts to confirm distances, etc. Recently I have been provided with a new AFM supplement that will allow me to calculate Unbalanced Field Length performance when required. I understand the definitions involved but cannot think of the circumstances that would drive me to abandon my FMS in favour of these new performance charts. Any suggestions?

[FE Hoppy]

whenever you want a real mans calculation rather than take off calc light from your fms.


But more seriously. Does your fms take into account obstacles and slope is it for the actual runway or a generic runway of similar balanced length?

With a full calculation you will find you can carry more load or choose lower or higher V1 as required even derate deeper to save money.

I'm surprised anyone uses balanced field calculations in these days of $200 net books capable of running full performance software.

[john_tullamarine]

whenever you want a real mans calculation rather than take off calc light from your fms

Prizes awaiting that man ... love it.

Supporting FE Hoppy's observations,

(a) balanced calcs are simplied and originally were desirable when the sums had to be done by hand using paper charts in the AFM. The answer was a reasonable weight and the time taken to get it was minimised

(b) unbalanced calcs take longer as you have to do each limit in turn. Unbalanced calcs generally will result in a higher RTOW than for balanced, depending on the particular aircraft matched to the runway environs geometry.

[galaxy flyer]

J_T

Why do some airplanes have V1 = Vr, whilst others inherently have V1<Vr? Two of my company's models V1 is always less than Vr while the largest one V1 always equals Vr on dry runways. Likewise, the B727, V1 = Vr except when on wet or contaminated runways. Assuming all of these examples lead to BFLs, not unbalanced ones. And yes, both of my examples could have V1 = Vr by unbalancing the field lengths.

GF

[john_tullamarine]

Why do some airplanes have V1 = Vr, whilst others inherently have V1<Vr?

For a general question, one can only talk around the subject.

The OEM can elect to schedule takeoff data in a manner best suited to marketing considerations while, at the same time, addressing certification requirements. As a result, one sees a range of presentations in AFMs.

Vr is driven largely by the time required for the takeoff flare to screen and achieving V2. As such it is linked closely to V2.

V1 is driven mainly by two considerations

(a) high enough to achieve a sensible distance to accelerate to Vr OEI ... which, in the limit, is Vr

(b) low enough to meet ASD requirements (while looking after Vmcg needs in the limit)

Apart from Types for which only balanced data is published (DC9 comes to mind) there will be a range of V1/Vr available for the operator to select for a given takeoff.

The typical, and available, range will be a consequence of how (a) and (b) mesh.

Two of my company's models

You'd be well placed to discuss the specifics with your aerodynamicist colleagues ?

Likewise, the B727, V1 = Vr except

Long time since I've flown the three holer and my AFMs are not to hand at the moment. However, I don't recall that being the case .. perhaps the operator you cite chose that as a flight standards policy ?

Sorry I can't be of much more assistance to the question ...

[Red Mud]

Thanks to all. Regarding my FMS, it does make its calculations based upon the actual aircraft weights, runway specifications and obstacle clearance. Therefore, unless things look marginal (TOFL, higher RTOW, etc) I am inclined to stick to the magic box and out of the AFM.

[mutt]

obstacle clearance Where does it get the obstacle information from?


Mutt

[Red Mud]

Hey Mutt ... I manually enter it into the performance data with information from the airport charts. Not everything is automated in my world.

[Checkboard]

The type of obstacle data required for a full performance calculation isn't available from airport charts.

You need to : contact each airport individually for the surveyed data up to MSA, out to 20nm odd, or sling a theodolite over your shoulder and go out and measure it , or subscribe to a company who does this for a living.

[john_tullamarine]

contact each airport individually for the surveyed data up to MSA, out to 20nm odd, or
sling a theodolite over your shoulder and go out and measure it , or
subscribe to a company who does this for a living. Given that (1) generally doesn't exist, (2) usually is impractical, a combination of operator research (there is a range of possible sources of useful data) and/or (3) ends up being the way to go ..

[mutt]

Red Mud, the answer from J_T is correct, however there are three approaches to the solution.....

1: FAR91 (Or equiv) Dont worry about those pesky obstacles....
2: FAR 121/135 hire a company to supply the obstacle data......
3: Follow the SID climb gradients....

Mutt

[galaxy flyer]

My present mob uses Aircraft Performance Group out of Castle Rock, Colorado. Ex-UAL group that provides runway analysis and provides the data within hours, if it is a oddball location. Normally, we do it online (I'll throw in a plug for ARINC's flight planning website, which offers runway analysis, fee required) or on a Blackberry. I have never run into a airport that wasn't in their database, but some our lot has and the answer was out in hours.

www.apg.aero

GF

[Pugilistic Animus]

I once ordered the free brochure,...now it's all online

[Pugilistic Animus]

Follow the SID climb gradients....


there's no money in that Mutt:}