Why does the MTOW increase with floats?

[Guest Andys@coffs]

Ive been around a while and the differences have been around longer than me... They are CASA imposed and given that legislative instruments and the really important exemptions that govern RAO's and recreational flying have been under "review" with a view to simplifying and consolidating many of these into one instrument have been "ongoing" for as long as I have I wont hold my breath for any "obvious" changes anytime soon......The wheels of progress move slowly in general and backwards in some aviation related empires.....

 

I remember Ozzie and me going hammer and tongs over the proposed Part 149 (I think it was) back in 2007/8, I was for it, he just wanted things left as they were....though glacial pace, neither of us needed to worry about anything in our lifetime it seems! Angst and passion wasted on the belief that an egg was about to be laid!!!

 

 

[ben87r]

Firstly, the views on MTOW being some sort or regulatory annoyance is scary and a safety issue.. Secondly, on limits, there needs to be some so there will need to be a line in the sand, don't like this? ATPLs and a A380 endo are available to all with enough $ and time. MTOW in both RA and GA as mentioned will be a limit to one or more factors which usually won't be structural of the fuselage or wings but may be stall speed limits GA has these also dependent on CAT, performance, GA also has this , and many others I'm not too aware of

 

Just because the wings don't fall off, doesn't mean it can't kill you

 

 

[facthunter]

Floats require a down force on the elevators to correct centre of drag change. This downforce is the same as added weight as far as the wings are concerned so the stall speed will rise. You may experience reduced elevator effectiveness to cause the stall which may give the appearance of being more docile, but may cause some difficulties with flaring for landing, too Nev

 

 

[jetjr]

stall speed and MTOW are linked with the design however in some cases the aircraft has structural capacity beyond the RAA max weight limit.

 

In most cases Ive looked into there is rarely much scope to increase MTOW without exceeding 45 kts stall limitation. Therefore raising one without the other doesnt achieve much for the current fleet.

 

Stall speed is the hard one to preserve in design terms i would have thought. Decreasing stall speed seems to have direct link with lower cruise speed too, so as always you get nothing without compromise.

 

Jabiru does have nice examples to look at having three different wings available on the same fuse/engine, with some MTOW to 760kg so declared stall speeds for weights well above RAA limits.

 

The overloading/safety issue is real and equally valid in RAA and VH segments. If you play outside these weights you will get burned one way or another eventually. BUT there is a difference in true MTOW and regulated (RAA) MTOW as exceeding the second will simply make the aircraft handle and perform differently, perhaps outside the capability of the RAA trained and certificated pilot.

 

 

[eightyknots]

"They sell you an aircraft that doesn't fit within RAAus rules by making you fly with half empty tanks to squeeze it into those rules."Zibi

 

This is not limited to RAA. Most GA aircraft (if not all) are fuel limited at MTOW if you fill all the seats and cargo areas.

Exactly, in a Piper Super Cub with two POB, we could only fill the tanks to half and we were at MTOW.

 

 

[Dafydd Llewellyn]

Again, I'm not talking about not being able to load max weight passengers, plus full fuel plus full baggage but rather this bit: J230 has a MTOW of 700 kg (VH Registration) and 600 kg (RAA Registration) (source: http://www.jabiru.net.au/aircraft-kits/j430-kit).

And that's something to complain about?

 

 

[Zibi]

And that's something to complain about?

Apparently:

 

The real question is why the regulatory MTOW isn't the same as the structural MTOW. I understand there has to be a line in the sand somewhere, but gimme a break. 150kt cruise, yep that's ok. CS prop, yep that's ok too. Oh you want to fill up your fuel tanks? Sorry, you'll need to paint a VH on the side to do that.Why not just have a list of approved aircraft? There's not that many!

[RickH]

How about the added drag from floats or do we assume that Vc, VNE is unaffected by the addition of floats.

 

 

[Dafydd Llewellyn]

How about the added drag from floats or do we assume that Vc, VNE is unaffected by the addition of floats.

Vc and Vne are STRUCTURAL LIMITATIONS. The only effect added drag has on them is the power (or dive angle) necessary to reach them.

 

 

[djpacro]

and so there is likely to be scope to reduce both (per FAR 23 for example) if you want to do it for some reason.

 

 

[ave8rr]

Why for example does a Taylorcraft BC-65 have a MTOW when a land plane of 1150Lbs but can go to 1228Lbs IF a seaplane?

 

Does this mean that the aircraft was certified to the higher weight? There is no mention of reduced speeds / G loadings etc in the seaplane config.

 

Looking at TCDS on the FAA web site for say a Cessna 180, an increased MTOW is also allowed when float equipped and is considerable (200Lbs or more)????

 

 

[Dafydd Llewellyn]

Good question. If it's in the TCDS, then those are the limits. How they got there, you'd have to ask the FAA.

 

 

[jetjr]

Could it be to do with full weight landing forces and fuse being supported differently on floats

 

Its an issue in ag aircraft i believe, ground, taxi and high weight landing are seriously damaging shock loads

 

 

[Dafydd Llewellyn]

Look, I'll say it again - the weight increase IS NOT the result of some magical physical effect of the floats that somehow makes the aircraft stronger. It's either the result of the original designers deciding to put out a float version of the aircraft, which they justified by analysis and/or test, for a higher weight. If Cessna advertise the 172 as a four-place aircraft, their sales Dept. presumably would want the seaplane version to also be a four-place aircraft - and to have anything like the same disposable load, the MTOW would have to increase. Standards such as FAR 23 and CAR 3 had an upper weight limit of 12,500 lbs (5700 Kg) so there was no regulatory "lid" on the MTOW; Cessna could add some additional structure and a larger diameter fine-pitch propeller, or whatever it took.

 

OR it's a subsequent modification, justified under the Supplemental Type certificate process, by either the OEM or a third party - probably the float manufacturer.

 

Either way, any weight increase has to be justified as complying with the original certification design standard.

 

When you come to the LSA rules, the MTOW the designer uses has to be pretty much the upper limit allowed by the category, or the aircraft will not be commercially competetive. The "lid" is set very restrictively, so the designer will inevitably be banging his head on it. So the writers of the rule decided to allow an extra 50 Kg for the designer to use, if he was designing a float version. It's for the DESIGNER to use, NOT something anybody can take advantage of by bunging floats onto an aircraft that was not originally designed for them. The STC process IS NOT AVAILABLE for LSA aircraft, because they don't have a TC in the first place, so no third party can come along afterwards and add a float kit to an LSA aircraft - it HAS to be built in to the original design.

 

So the title of this thread is misleading - the MTOW of an LSA aircraft does NOT increase with floats - unless that was designed into the aircraft by its manufacturer.

 

With a certificated aircraft, the STC process is available; and that's very likely the way most GA float conversions were approved.

 

 

[jetjr]

Yep agree, on an TC aircraft, all understandable

 

LSA all understandable too, effectively similar rules, ie no changes without someone qualified say so .

 

Confusion comes on others where builder can fit floats to whatever they like and claim larger regulatory MTOW. Different ceiling for that designer.

 

For example a kit built J250, registered 19 RAA, i can fit float kit from anywhere, test for stall <45 kts and claim extra 50 kg MTOW. Airframe is ok to 700+ kg.

 

Point some are trying to make is this example doesnt make sense, why not same rules for everyone. Why not exemptions for better instrument pack, extra fuel, other safety gear

 

 

[Dafydd Llewellyn]

Yep agree, on an TC aircraft, all understandableLSA all understandable too, effectively similar rules, ie no changes without someone qualified say so .

Confusion comes on others where builder can fit floats to whatever they like and claim larger regulatory MTOW. Different ceiling for that designer.

 

For example a kit built J250, registered 19 RAA, i can fit float kit from anywhere, test for stall <45 kts and claim extra 50 kg MTOW. Airframe is ok to 700+ kg.

 

Point some are trying to make is this example doesnt make sense, why not same rules for everyone. Why not exemptions for better instrument pack, extra fuel, other safety gear

An RAA -19 aircraft is the equivalent of a GA Experimental amateur-built aircraft; provided it meets the requirements of CAO 95.55.1.5 - i.e. the major portion rule, and the category stall speed limit, etc, YOU are in effect the designer. The fundamental principle of the experimental provisions in CASR Part 21 is 180 degrees opposite to the fundamental principle of all other aircraft; in regard to all other aircraft (ALL aircraft, prior to the introduction of CASR Part 21. 191 thru 21.195, in 1998) the fundamental principle is that CASA protects people from their own stupidity and ignorance. For aircraft that come under CASR 21.191 and CAO 95.55.1.5, the principle is voluntary acceptance of risk. If you want to understand this better, look at CASA AC 21.10.

As George Markey was wont to say - you can make an experimental aircraft out of jelly and custard - CASA can't stop you. What they CAN do is decide who can fly it, where, and when. (Probably McMurdo Sound, in a blizzard, in that case). Presumably RAA can make similar constraints, tho I do not know if they have ever recognised their responsibility in this area.

 

So this freedom comes with strings. Certificated aircraft have different strings, and LSA aircraft have yet another set of strings. A chaque un, son gout.

 

 

[jetjr]

Yeah that was interesting, they did lots of work developing "safer" airframe and FAA recognised it which is good I guess.

 

Keep in mind I think in the US regulators are chartered to promote aviation too - stopping the approval of a new aircraft, proving to be very popular, possibly getting more people into flying, and NOT recognising efforts to advance safety in design would go against this

 

I expect in Aus that stopping more aviation participants would be a just bonus

 

 

[Oscar]

I think this might represent a breakthrough; the FAA has (apparently) accepted that there are grounds for acceptance of aircraft that do not conform to a non-FAA standard ( the ASTM LSA standard) into the LSA class. The whole MTOW thing has always been an arbitrary number imposed as an intellectually lazy way of defining a performance envelope (not dissimilar to speed limits being defined as a 'safe/unsafe' determinant with no acceptance of the driving environment actually existing in any specific case).

 

We could hope that this will filter down to individual national airworthiness authorities (read, for us, CASA) accepting that an aircraft that utilises a greater MTOW than defined in the ASTM standard for the inclusion of greater safety (particularly for structural improvements, rather than extending 'performance'). That could lead to very considerable improvements in things such as crash-worthiness, repair cost, insurance premium cost etc. for useful aircraft that still remain fundamentally LSA-class aircraft.

 

We have all seen evidence of the lack of crash-worthiness of LSA-class aircraft that have been built to extract the maximum performance within the restrictive LSA standards. A carbon-fibre composite cabin structure simply explodes on severe impact like a smashed bottle, whereas a lower-tech but heavier cabin structure can absorb the same sorts of loads with a resilience that provides a far better chance of minimal injury.

 

We have seen the weight of passenger vehicles increase with additional occupant-safety measures quite considerably over the last, what - 10 - 15 years? It's time that aircraft regulations accepted occupant safety measures - just for a start - as valid reasons to accept increased MTOWs. The operational environment in a country like Australia (or probably also South Africa) is considerably more demanding than for much of Europe: gust loads, outback airstrip conditions - even distance between airstrips etc. We need rugged aircraft, more fuel, strong landing gear, big wheels etc.

 

The ASTM standard requires that 'one size fits all'. It should be regarded as a minimum set of characteristics - not a box into which everything has to fit. It appears that he FAA has broken through the mind-set of blind obedience to that standard; I'd like to think that CASA could embrace the same idea. Perhaps with the replacement of McCormick as head of CASA, we might just see that happening.

 

 

[eightyknots]

I think this might represent a breakthrough; the FAA has (apparently) accepted that there are grounds for acceptance of aircraft that do not conform to a non-FAA standard ( the ASTM LSA standard) into the LSA class. The whole MTOW thing has always been an arbitrary number imposed as an intellectually lazy way of defining a performance envelope (not dissimilar to speed limits being defined as a 'safe/unsafe' determinant with no acceptance of the driving environment actually existing in any specific case).

The same thing has occurred with the Terrafugia flying car which fits into the LSA category. Back in July 2010 it was announced that the Terrafugia Transition was granted an exemption from the FAA concerning its MTOW. The Transition may be certified with a take-off weight of 650 kg; this limit matches the MTOW for amphibious LSAs mentioned in this thread.

 

Why the extra 50 kg? The FAA granted the extra weight allowance so that Terrafugia could fit the mandatory road safety features (eg airbags and bumpers) into the Transition car/aeroplane.

 

For information about the $AUD300,000 car/plane, see: http://www.terrafugia.com/sites/default/files/TransitionSpecSheet-July2013.pdf