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CAO 20.7.4 and answering take-off and landing weight charts that include a slope

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GeeEss created the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

Hey all,

I've read volume 1 and the recommended reading on vol 2 while preparing for my RPL exam.

I also use another resource for pilots that want to practice some exams. While working though the questions, I noticed that there was an added twist that I had not encountered before. It was regarding the inclusion or omission of a slope greater that 1% in the working out of the landing/take off charts and it references CAO 20.7.4.

After reading the order, I'm still very confused and was hoping someone could extend their expertise and knowledge with me regarding this matter.

Any help would be appreciated.

Thanks in advance!
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  • John.Heddles
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John.Heddles replied the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

Your post doesn't explicitly state just what your concern is so I can only speak generally.

This slope thing for light aircraft has been around for a long time - if I recall correctly (in the absence of spending a lot of time digging in the files to see if I have any copies of the old ANOs) the following considerations are relevant

(a) with the regulatory culling post-Yates about all we have to go on is CAO 20.7 which is an operational CAO and, for this consideration, imposes an operational procedure for figuring weights and distances

(b) if I recall correctly, much the same words existed in the ANOs when I started out in the game during the 60s

(c) this strongly suggests that the original introduction of the words (the exact origin would be an Herculean task to track down, I fear, and, unless someone out there has some very old files, would require unrealistic co-operation from CASA to waste their time in the ancient, dusty archives) was co-incident with the routine operation of ancient rag-bag machines such as Tigers, Austers and so forth (all lovely birds so I use the rag-bag term only with great affection).

Back in those days, it was common for folks to operate from paddocks and other convenient locations which had little resemblance to the aerodromes and airfields with which most of the readers would be familiar. One of the problems with that (and most of us oldies have done plenty of paddock flying so we are well aware of these problems) is estimation of distances, slopes and gradients.

I suggest (and it's just an educated guess after years of flying and aero engineering work) that the 1% slope words reflect a concession which acknowledges that shallow slopes don't have a major impact on the performance calculations and can be considered subsumed into the takeoff and landing fudge factors which appear elsewhere in 20.7.4. Further, back in those days, many light aircraft operated on the basis of generic strip length requirements rather than having specific takeoff and landing charts in the flight manual or CofA.

Now, I can only speak as both an experienced pilot and experienced engineer (the latter with considerable performance experience) - the legal folk may well read specific legal requirements into the words. With that caveat (and the caveat in the next paragraph), I suggest that the words provide a baseline for calculations but don't preclude the use of slope in the calculations if you have both the slope data and charts which account for slope.

Operationally, if you have charts which account for slope, and you have the slope data for the runway, you would be a bit silly not to consider the slope. Be cautious though, that you don't apply a conservative slope and, as a result, come in under the basic conservative 20.7.4 calculations. Certainly, however, if the slope on the day provides a non-conservative weight figure, then you would be silly in the extreme not to account for the slope effect and operate to a weight a little less than that permitted by the Order. Similar thoughts apply for surface corrections (which address consideration of the variation in rolling/braking coefficients of friction).

If this doesn't address your concerns, perhaps you can add to the discussion and we can try again ....

To save folks the trouble of digging out the words in the Order, we are talking about

4.1 (a) “ … runway slope (if in excess of 1%) … “
5.1 (a) “ … runway slope (if in excess of 1%) … “
6.2 “ … take-off distances are to be determined for a level short dry grass surface … “
10.2 “ … landing distances are to be determined for a level short dry grass surface … “

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Stuart Tait replied the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

As far as the exam is concerned you will not get a slope less than 1% not including 0 so if the slope is 1% or over use it in the charts

Cheers
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bobtait replied the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

That issue has been a drama ever since I got involved in teaching.

I did ask CASA what their interpretation was, they said they would make it clear in an exam question by including a phrase like "having regard to all of the circumstances, including slope". Otherwise they will avoid questions where the slope is less than one percent.

How hard would it be to simply delete reference to the 'less than 1%? In the hundreds of times I've read that clause, I'm still not sure whether it means you may not use the slope if it is less than 1% or you must not use the slope if it is less than 1%.

If you've got it - use it. Wish we could just get back to flying aeroplanes!
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John.Heddles replied the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

If you've got it - use it.

Philosophically, as a performance engineer, I couldn’t agree more.

However, we need to make sure that we don’t expose ourselves, needlessly, to CASA censure and, far more importantly, to real legal censure after the accident where folks are injured or killed. Again, the caveat is that I have no legal expertise so my comments need to be read with that in mind – the lawyers might just come up with a different interpretation.

4 Take-off weight limitations

4.1 An aeroplane must not take off at a weight in excess of the least of the weights determined in accordance with subparagraphs (a) to (d):

(a) a weight at which the take-off distance required under subsection 6 for the pressure height, temperature, runway slope (if in excess of 1%) and wind component along the runway, is equal to or less than the take-off distance available in the direction of take-off. Approved declared conditions may be used instead of actual pressure height and temperature;


This ties it in with S6 so we need to read both in conjunction. Never mind that the permission to use declared conditions makes it all a bit of a joke when it comes to splitting hairs.

There is a clear requirement to consider slope if it is greater than 1% but I see no inherent prohibition on considering slope for shallower runway gradients.

6 Take-off distance required

6.2 For aeroplanes operated on land, take-off distances are to be determined for a level short dry grass surface. For aeroplanes operated on water, take off distances are to be determined taking into account the maximum crosswind component and the most adverse water conditions for the aeroplane type.

6.3 Where there is an approved foreign flight manual or a manufacturer’s data manual for an aeroplane that sets out the take-off distance required for that aeroplane, then that aeroplane must be operated so as to comply with either the requirements set out in paragraphs 6.1 and 6.2 or the requirements relating to take-off distance set out in either of those manuals.


I suggest that the silliness of the discrepancy between S4 and S6 regarding slope reflects only the fact that the Regulator has never bothered to tidy up the discrepancy. Without digging into the archives, my recollection is that it has been there for a long time.

It is not just the slope consideration; the variation in surfaces (read presumed/assumed rolling coefficients of friction) can have a marked effect on the numbers which come out of the calculations.

Use of the OEM data, again and potentially, makes for a concern. Keep in mind that there is a distinct difference (which can be quite significant in the performance arena) between “an approved foreign flight manual” and “a manufacturer’s data manual”. The former (which should be “good gen”) has a tick from the Regulator (typically the FAA for most of the Australian fleet) while the latter is commercial information only which may, or may not, have much tie-in with the Design Standards. This latter consideration probably is only a concern for much older aircraft which predate the general introduction (quite a few years ago, now) of the GAMA Spec No.1 format POH documents (which include both classes of document).

As one might see on very much older nautical maps, “Here be dragons”.

I suggest that what this means is that one should only consider slopes and surfaces to the extent that such imposes a penalty compared to the S6 requirements. That is to say, including uphill slopes (which will increase the TODR) and other than sealed surface data (likewise) ought not to be a problem. However, I see a potential adversarial contention if you go the other way ….

5 Landing weight limitations

5.1 Except in an emergency, an aeroplane must not land at a weight in excess of the least of the weights determined in accordance with subparagraphs (a) and (b):

(a) a weight at which the landing distance required in accordance with subsection 10 for the pressure height, temperature, runway slope (if in excess of 1%), and wind component along the runway at the time of landing, is equal to or less than the landing distance available in the direction of landing. Approved declared conditions may be used instead of actual pressure height and temperature;


Similar comments to those before …

10 Landing distance required

10.2 For aeroplanes operated on land, landing distances are to be determined for a level short dry grass surface. For aeroplanes operated on water, landing distances are to be determined on flat broken water.

10.3 Subject to paragraph 10.4, where there is an approved foreign flight manual or a manufacturer’s data manual for an aeroplane that sets out the landing distance required for that aeroplane, then that aeroplane must be operated so as to comply with the requirements set out in paragraphs 10.1 and 10.2 or the requirements relating to landing distance set out in either of those manuals.


Similar comments to those before …

In the landing case, we ought not to have a problem using downhill slopes – surface can be a bit more of a concern but can be figured by inspection of the landing chart data.

Perhaps Bob might refer the question to his legal man for comment ? I would be interested in hearing what the legal side of the house might have to offer on the matter.

As to how this should affect the exams ? Beats me, folks – as always, just try to fly the check captain (examiner) on the day. The poor old examiner has the same problem - he has to do his best to incorporate whatever the rules might mean into his exam questions.

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Carello replied the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

I suggest that the silliness of the discrepancy between S4 and S6 regarding slope reflects only the fact that the Regulator has never bothered to tidy up the discrepancy. Without digging into the archives, my recollection is that it has been there for a long time.

Thanks for clearing up the above. I thought I was the only one seeing the elephant in the room.
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John.Heddles replied the topic: CAO 20.7.4 and answering take-off and landing weight charts that include a slope

You, good sir, as another one of us olde folkes, would remember back to the days of the ANO/CAO 101 series which addressed the airworthiness requirements, albeit that, in the main, that was just to call up the foreign rules and add a bit of fluff here and there.

There were more than a few discrepancies between what the airworthiness engineering and certification folks (who were acknowledged experts in their fields - the likes of fine chaps such as Icko, Ron and their contemporaries) wrote in 101 and what ended up in the operational stuff in the 20 series. The operations folks certainly were fine chaps, as well, but they were, in the main, expert pilots who knew all about piloting stuff but, not always, enough about certification and like matters.

The main concern I have with the demise of the 101 words is that the aggregate corporate knowledge which went along with their content was thrown out with the bathwater, post Ron Yates' Report's consequences. That, in consequence and subject to what the legal folks' interpretation of words might be, leaves the new chums at risk of glossing over the syntax and, perhaps, missing the odd point which might leave them exposed to censure through no culpable fault of their own ....

I think most of us who have had feet in multiple rooms of the house see the occasional elephants in the room.

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