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WHEN IS AN OBSTACLE NOT AN OBSTACLE?

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WHEN IS AN OBSTACLE NOT AN OBSTACLE?

Old 25th Oct 2001, 05:48
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Question WHEN IS AN OBSTACLE NOT AN OBSTACLE?

WHAT IS CONSIDERED AN ACCEPTABLE MARGIN FOR ERROR WHEN INVESTIGATING OBSTACLES FROM DIFFERENT SOURCES BEARING IN MIND IN ITS SIMPLEST FORM THIS DATA IS PRODUCED OFF A CHART AND ACCURACY IS DIRECTLY RELATED TO THE AMOUNT DRUNK THE NIGHT BEFORE WHEN USING DIVIDERS?
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Old 25th Oct 2001, 12:58
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This question ought to generate some discussion.

One should review the basics first -

(a) the rules in a particular jurisdiction are what the appellate court(s) judge(s) determine(s) them to be after the inquiry into the inquiry (ad infinitum) into the crash.

(b) apart from (a) the purported rules for a given operation will require the consideration of matters such as

(i) the plan area to be considered for obstacle clearance. This normally is some sort of quadrilateral-derived shape and is superimposed over the intended aircraft flight path. The actual shape varies from Standard to Standard.

If a lumpy bit is inside the area, it is to be considered, if outside, it WILL be ignored.

As a result, the pilot MUST track the nominated escape path (within the nominated speed and bank angle range) with critical precision, as some runways may have quite extreme obstacle hazards IMMEDIATELY outside the (rather narrow) plan area to be considered - you go outside - you might very likely die. This is especially the case where the ops eng guys call for turns to avoid terrain.

(ii) the calculated net flight path (ie expected OEI performance less a specified fudge factor) is then compared to each obstacle height plus a small margin and, if it is above the obstacle height so calculated, the obstacle can be ignored. Otherwise either the RTOW reduces or the ops eng guy looks for a different escape path.

So, to your question -

(c) Each source of data will have a inherent and able to be determined accuracy probability. There are various approaches to addressing such errors in ops eng calculations

(d) The error probability will vary according to the means with which the data has been determined, especially if photographic based.

(e) The error probability then, itself, increases with time from the date of data acquisition involved in the production of the final data. For instance, a map can only be as accurate as the original aerials and any subsequent field checks used in its construction, and then only up until the date(s) of those activities

(f) maps generally give ground heights, not tree heights, for instance... and the vegetation changes constantly.

.. the problems just go on and on ...


As an ops engineer, my attitude is

(g) I do the best I can considering the time and data available

(h) I apply additional conservatism if, and as, I consider it appropriate

(i) if I have time and the need is appropriate, I will consider a specific survey of critical obstacles

(j) I endeavour to have the operator's flight standards organisation institute an ongoing flight deck monitoring of the general area in the escape path


What does that mean in terms of your question and my original point (a) ?

I suspect that no matter what you or I do, the lawyers will always try to find fault with it.

It follows that NO level of error is acceptable.

It is a matter of deciding what you, as an operator, pilot, or engineer, can live with in an environment of inaccuracy. Subsequently you can be called on to justify your decision(s) and be liable to penalty if the legal system decides that your decision was not quite up to the mark.
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Old 25th Oct 2001, 16:30
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noflypaul,

Despite my friend JT's view, it gets down to what is reasonable in the circumstances, or the somewhat different animal "not unreasonable", at least in jurisdictions sourced in British law.

The judge may publish reasons that form a precedent ("ratio decidendi") or may make non-binding observations ("obiter dicta"), but they relate only to the statute as then applicable. Should there be a judicial interpretation that was unintended by the legislature (read regulator), there is normally a very rapid amendment to the controlling statute. Hence, there is much more structure and certainty than JT's post might otherwise infer.

Now, to your question...

Start off with the statutory requirements and any amplifying documents published by your State's regulator. If there is little in the public arena, there is sure to be internal records of the deliberations that led to the statute.

If all of that reveals nothing (the most common result) then you have to conjure up a risk assessment that is "defensibly conservative", ie has a level of conservatism that identifies and accounts for each risk without unnecessarily killing payload. A good example occurs when determining the take-off splay: if you have a choice between FAA (300 feet either side of centreline) or ICAO (90 metres either side expanding at 12.5% to 600/900 metres) or Australia (250 feet either side expanding at 12.5% until reaching LSALT), try playing with the arguments for and against.

As far as obstacles determined from a map or chart, then common practice is to establish the vertical and horizontal charting tolerance and then move the obstacle by the appropriate amount closer and higher to the take-off point. If you are dealing with a contour line, then you take the next highest contour level. Then you should apply a vegetation tolerance appropriate to the local trees.

And when all that hurts the payload too much, pay for a surveyor to get rid of the conservatism and get down to reality, at least close in.

Local environmental effects and aircraft handling are generally very difficult to cater for.

Have fun
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Old 25th Oct 2001, 17:09
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And some operators just do their own Type "A" surveys. It might be overkill, but it gets rid of doubt about obstacle elevation, chart error and vegetation growth. That's always better for payload, in the long run, and is more readily defensible at law, should the need arise.
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Old 26th Oct 2001, 00:57
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NoFlyPaul

I’m guessing that what you are trying to do is compare obstacle data from two separate commercial suppliers. I have never dealt with your normal supplier, but the other company uses AIP charts and an optical scanner to plot the position of the obstacles, these charts go out to a maximum distance of around 12 nm’s. (I can hear JT starting to scream at this stage….. )

I would presume that your normal supplier uses a pretty similar system. It should therefore be relatively easy for you to obtain AIP charts for the airports that you want to review and plot the obstacles yourself. (Try www.iata.aodb.org to download AIP charts.) Alternatively you could get the information from a topographical map of the airport area. The margin of error that you are looking for is ZERO.

The data that either supplier uses is only as good as they receive from the regulatory bodies. I have seen AIP obstacle data that was mixed up and gave the obstacles for the opposite end of the runway. So you will need some form of “human” review of this data before you start to use it operationally.

J_T and 4dogs, that’s actually the first time that I have heard about making amendments to the spot heights for vegetation. Our newest route is going to be to Peshewar (sp) in Pakistan, there is no way on earth that I’m going to stroll around this airport looking at the plant life……….

4dogs, Australia (250 feet either side expanding at 12.5% until reaching LSALT),. Is there a maximum splay width?


Mutt

(Edited to add smiley things..)

[ 25 October 2001: Message edited by: mutt ]
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Old 26th Oct 2001, 02:41
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Thanks for all the input, I shall go and lie down in a dark room and curse my big mouth for opening it in the first place.
Mutt is quite correct I am trying to check one set of commercial data from a finished performance product against another set of obstacle data to proove the accuracy of the first (amongst other things). The idea being if the input data is correct and the software works then output data cannot be wrong!
This might be a simplistic attitude but I am a one-man-band with my own job to do apart from this "hot potato". Once the checking program is complete I hope to hand it to operations and let them deal with it.....
I was working on 1ft elevation and 30ft length as my margin for error (someone gave me those figures somewhere in the dim and distant past) but even using that I am still finding discrepancies. Both suppliers claim to be using the same source data in the same way.
I will sign up for the IATA database and see where that leads me.
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Old 26th Oct 2001, 17:53
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think twice bout using IATA for electronic copies of AIPs.
AIPs will generally only present obstacles for a straight out procedure. What about when u want to use an emergency turn procedure to maximise your payload/flex.

where are you going to get your obstacle information then?

a while back i did an extensive investigation of finshed product providers, and data providers.
LIDO seems to be the best data provider, Jeppesen came second.

for finished product providers check out www.flygp.se

O\z
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Old 26th Oct 2001, 20:29
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R4D and I like to joust .. or am I just an incurable cynic in my views ? ... certainly I am of the committed view that EVERY decision made is subject to subsequent legal scrutiny and censure. A cursory review of some legal decisions over the years in Australia alone gives chilling food for thought.

I guess that what I was saying in a round about way was that the interpretation which counts is the judge's after the event. In Australia, for instance, it is often very difficult to get some elements within CASA to commit themselves to an interpretation of their own regulations. Certainly it is my understanding that CASA personnel are very wary of the judge's subsequently holding their interpretation to be invalid.

Apart from general PD reading, I have no legal competence, only considerable wariness.

Mutt,

I would be checking the basis of the charts you use. Those in my ken have ground level spot heights, to which one has to add a vegetation allowance.. and there are some big trees out there. Charts may also include heights of specific cultural objects .. towers and the like.

noflypaul,

Don't worry too much. If you get involved in this sort of thing, you must expect to get headaches ... why should you miss out on the fun ? In fact, doing the number crunching is the easy bit .. getting good obstacle data is the difficulty.

I have very jaundiced views of some commercial services. Prudence dictates that I choose not to discuss such things further in this forum.

Trying to do an independent check is very difficult. First you have to get the provider to define the obstacle set used. Then, when you determine your own, it is likely to be different. Unless you input the same data set, why expect to get the same answer ?

It is all very much dependent on the sources of data. A difficulty facing the larger providers is one of locating local data which is more accurate than that which is readily available ... this takes local contacts and knowledge. Much of the really useful data is not public domain and can be extremely difficult to locate. We all develop our appropriate contacts in the geographical area of operation. I end up with data from sources such as mining companies (often very useful), water supply and other local authorities, and so forth. If needs be ... out in the 4WD and take a look for oneself. Sometimes hire a light aircraft and do an aerial inspection to narrow down the specific areas of interest which might need further investigation.


At the end of the day, you pays your money and you takes your chances.. and, to do a good job, it takes resources and time .. and that costs dollars. The crunch comes when the crunch comes and you have to justify your actions and processes.

[ 27 October 2001: Message edited by: john_tullamarine ]
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Old 8th Nov 2001, 16:05
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Mutt,

Sorry about the delayed response - just been off arguing with a bluddy "expert aviation surveyor" about the need for full obstacle assessments rather than his view of the single critical obstacle based on gradient. Arrrgh...

To answer your question about the Australian take-off splay: Until very recently, no. The rationale is that, industry wide for all types above 5700 kg, the aeroplane and crew do not have infallible means of assessing drift and therefore do not know when they have achieved 600 metres off track when DR'ing in the gloop.

After the regulator and a number of operators got burnt by a certain very large commercial supplier who, as best I can figure, never provided compliant Australian data - quelle surprise - they recently introduced an option to adopt the ICAO splay. This is, plus surprise, an option from that great icon who must not be mentioned.

JT, at best I was only expanding a little on your theme - I wasn't really chiding too hard. See you at the end of the month?

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Old 9th Nov 2001, 18:59
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G'day R4D,

I argued along the same lines with CASA some years ago about splays used by commercial providers... and about one in particular... which is, quite likely, the one to which you refer ?

I'll be back in country for a few days in the next week or so but have to head off again prior to the 30th so the Bend is a no show for me. Do give me a call via email and a beer would be in order if our paths can be made to cross in the week or so prior.

[ 09 November 2001: Message edited by: john_tullamarine ]
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