G-METER question (non TSO) - can I swap 3" unit with a 2"?

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G-METER question (non TSO) - can I swap 3" unit with a 2"?

#1 Post by cgzro » Sun Jan 31, 2016 2:15 pm

I have a 3.25" Century G-METER in my certified plane. I'd like to replace the 3" unit with a 2" unit to make some room for a new engine monitor (GEM G2). The question is can I replace the larger 3" G meter with a smaller G meter. I note that there is no TSO for a G meter.

Anybody know or can point me to any pertinent references?

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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#2 Post by Big Pistons Forever » Sun Jan 31, 2016 5:26 pm

Page 2-9 (limitations) of the Pitts S2B manual I have lists the required instruments. It only specifies a " G Meter " with out any further details as to type or size. The definitive answer would be in the factory maintenance manual and parts catalog.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#3 Post by photofly » Sun Jan 31, 2016 10:09 pm

Have you done anything to piss off Transport recently? Is a TC inspector going to complain?
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#4 Post by cgzro » Mon Feb 01, 2016 5:35 am

I contain my bad behaviour to one cubic kilometre over gatineau so I assume TC is ok with it :) .. and I try very hard to be totally legal although its hard work sometimes

The Parts manual says 'Accelerometer, Aircraft: AN5745-2"

So I guess I need to find an equivalent Army Navy Part since they are not TSO'd.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#5 Post by PilotDAR » Mon Feb 01, 2016 6:29 am

You must be getting younger Peter. All the rest of us are getting older, and would like the instrument faces to be larger!

G meters are a little hard to calibrate. Just for the technical records, I'd take both instruments for a flight together, and make a correction table between them, adding in the 2G turn. With that, you have a record of "checking" the airworthiness of the G meter. Beyond that, I agree with Photofly, will anyone worry about it?
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#6 Post by cgzro » Mon Feb 01, 2016 12:53 pm

So I could put a non TSO non AN part and simply do a calibration W&B and thats ok?

Im so used to all the requirements for STC / TSO that Im surprised (but happy) it could be so simple?
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#7 Post by photofly » Mon Feb 01, 2016 9:49 pm

It feels different because it doesn't connect to anything in the aircraft. Similar to how it would be to replace one aircraft clock with another. YMMV.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#8 Post by torquey401 » Mon Feb 01, 2016 11:07 pm

Is a G-meter required equipment? What does the equipment list show? What does the basis of certification say?

Interesting question.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#9 Post by cgzro » Tue Feb 02, 2016 5:19 am

Ok, so the equipment list says and I quote:

"16. Accelerometer (Optional) AN 5745-2 or equiv) ..."

So presumably I could completely remove it if I wanted .. which I don't.

I guess the question them becomes is any non TSO'd Accelerometer is "equivalent" to an AN5745-2?
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#10 Post by SuperchargedRS » Tue Feb 02, 2016 7:21 am

I would say yes.

I also think you're over thinking it, swap the units and carry on.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#11 Post by photofly » Tue Feb 02, 2016 7:37 am

To what extent do you rely on an *accurate* g-meter reading when flying? More pointedly, what are the foreseeable consequences to you if the thing is incorrect by ±0.2g, ±0.5g, ±1g, ±2g? The answer to that would inform me as to how picky I need to be about what I install. I might be more concerned about real consequences than paperwork consequences.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#12 Post by cgzro » Tue Feb 02, 2016 8:38 pm

Me personally? almost never. I've been doing aerobatics for so long now that speeds/radius/pull force etc. are kind of ingrained and frankly there is no need to go to the G limits of the plane because it scrubs off so much energy its counter productive. However for anybody new to the plane, especially somebody young and a bit more aggressive its important to see the numbers because you can easily blow past 6G if you are not careful, so I'd never outright remove it.

So I suppose I'd want the meter to be accurate to say 1/4G or so at the limits (+6/-5). Should be kind of fun to calibrate. I guess I could strap the new one somewhere and put a camera on both and fly around and see how they react relative to each other ;)

Structurally I worry more about rate of change of G (a.k.a "Jerk") as its very hard on things even when you stay within the limits.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#13 Post by hoptwoit » Tue Feb 02, 2016 9:32 pm

If the unit is marked as optional can you not just go down the major or minor list. If the question is no to all questions then sign it off as a minor mod replacement component. I went through this once with transport hard mounting a handheld GPS and wiring it into the aircraft avionics buss. Classified it as a minor mod put a big placard on the panel." VFR only" and don't interface it with anything else. entered in logbook as minor mod and Transport Inspector was happy. These units were non TSO. As with all things aviation opinions would vary but so far so good. Hope this helps.

(1) General

The following criteria outline a decision process for assessing the classification of a modification or repair.

Information Note:

For each issue it shall be determined whether the modification or repair to be accomplished could have other than a negligible effect on those characteristics contained in the definitions of "Major Modification" and "Major Repair", pursuant to section 571.06 of this standard. The following questions are answered with either a YES or NO response. A YES answer to any individual question indicates that the modification or repair shall be classified major.
(2) Criteria

(a) Operating Limitations

Does the modification or repair involve a revision in the operating limitations specified in the approved type design?

(b) Structural Strength

Information Note:

The questions contained in this paragraph shall be applied to alterations of an airframe, engine, propeller, or component.

Does the modification or repair alter:

(1) a principal component of the aircraft structure such as a frame, stringer, rib, spar, skin or rotor blade?

(2) a life-limited part or a structural element that is subject to a damage tolerance assessment or fail-safe evaluation?

(3) the strength or structural stiffness of a pressure vessel?

(4) the mass distribution in a structural element?

Information Note:

This might involve the installation of an item of mass that would necessitate a structural re-evaluation.

(5) a containment or restraint system intended for occupants or the storage of items of mass (e.g. cargo)?

(6) the structure of seats, harnesses, or their means of attachment?

(c) Powerplant Operation

Does the modification or repair:

(1) affect the power output or control qualities of the powerplant, engine, propeller, or their accessories?

(2) alter the approved operating limitations?

(d) Performance and Flight Characteristics

Does the modification or repair involve alterations that:

(1) significantly increase drag or exceed aerodynamic smoothness limits?

(2) significantly alter thrust or power output?

(3) affect stability or controllability?

(4) induce flutter or vibration?

(5) affect the stall characteristics?

(e) Other Qualities Affecting Airworthiness

Does the modification or repair:

(1) change the information on, or the location of, a placard required by the type design or an Airworthiness Directive?

(2) alter any information contained in the approved section of the aircraft flight manual or equivalent publication?

(3) affect the flight-crew's visibility or their ability to control the aircraft?

(4) affect egress from the aircraft?

(5) reduce the storage capacity of an oxygen system, or alter the oxygen rate of flow?

(6) affect flight controls or an autopilot?

(7) alter an electrical generation device, or the electrical distribution system between the generating source and either its primary distribution bus, or any other bus designated as an essential bus?

Information Note:

The electrical distribution system includes its associated control devices, and all its protection devices.

(8) reduce the storage capacity of the primary battery?

(9) affect a communication system required by the approved type design?

(10) affect instruments, or indicators that are installed as part of a system required by the approved type design?

(f) Other Qualities Affecting Environmental Characteristics

Does the modification or repair increase aircraft noise levels or emissions?

amended 2002/06/01)
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#14 Post by CID » Wed Feb 03, 2016 10:07 am

It's a little tricky when replacing gauges but lack of TSO doesn't matter as much as making sure you "qualify" the new gauge for your particular installation. Look for specifications that were developed using accepted critiera. DO-160 is good and for G meters specifically, one made to MIL-A-5885C would come with the applicable specifications.

If the specifications are within the environmental envelope of your aircraft instrument panel, you're in goo shape.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#15 Post by Big Pistons Forever » Wed Feb 03, 2016 6:40 pm

cgzro wrote:Ok, so the equipment list says and I quote:

"16. Accelerometer (Optional) AN 5745-2 or equiv) ..."

So presumably I could completely remove it if I wanted .. which I don't.

I guess the question them becomes is any non TSO'd Accelerometer is "equivalent" to an AN5745-2?
The POH requires a G meter for aerobatics, so if you removed it you could fly the airplane but not do any aerobatics. It would seem to me the "or equivalent" statement would be the let that would allow you to install a smaller G meter. I guess at the end of the day the only thing that matters is whether or not your AME is willing to sign out the swop.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#16 Post by DonutHole » Thu Feb 04, 2016 5:18 pm

I would probably answer no because analog g-meters don't usually require power... but I'm probably just talking out of my hind.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#17 Post by all_ramped_up » Thu Feb 04, 2016 6:06 pm

Now I'm no ACA but from my knowledge of G-Meters, methinks that hoptwoit pretty much answered it.

I'll have a peek at the G-Meter in the jet tomorrow but I l am 99.536% sure it isn't powered as it's analog as well. The hands are luminescent so there would be no reason for external connection to any bus. So according to the CARs, it qualifies as a minor mod. At least that's how I'd call it.

My rare $0.02

edit -

Just looked in the AMM...

"The G-Meter operates on the principle of inertia and is independent of electrical sensing systems. A moving mass is displaced in the direction of the accelerating force. The displacement is converted into a rotary movement which is restricted by a spring."

And wrt checking calibration of the new unit... you could try this:

1) Hold G-Meter so the scale is vertical and that reset knob is on the bottom left hand side. Should read +1G +/- 0.2
2) Hold G-Meter so the scale is horizontal. Should read 0G +/-0.2
3) Hold G-Meter so the scale is vertical and the reset knob is on the top right hand side. Should read -1G +/-0.2
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#18 Post by CID » Thu Feb 04, 2016 7:00 pm

Not a minor mod. It's required equipment for certain ops. 571 is quite vague on this point but required equipment is in the realm of "major". Even if it wasn';t where would you find acceptable data for the installation of a G meter?
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#19 Post by photofly » Thu Feb 04, 2016 7:06 pm

If you can tell us what "acceptable data" would look like, I'm sure we can find it.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#20 Post by PilotDAR » Thu Feb 04, 2016 10:19 pm

Hoptwoit's reference to STD 571, Appendix A is appropriate, and useful, however the installation of a G meter would slip between the cracks in it. For myself, I would not tend to find that I need to issue an STC for the replacement of one G meter with another, if they were both MIL Spec or TSO'd, and the aircraft parts catalog said "or equivalent". I'm not certain that a VFR Pitts would require TSO'd equipment, as long as the proposed instrument was AN, MIL Spec, or "equivalent". G meters do have a special niche in this though, as they do indicate a value which is subject to limitation in the aircraft type design, so it should be "equivalent", and airworthy. I would be surprised if an inspector or AME would have issue with an airworthy G meter change within the meaning of "equivalent". But that determination would be their job, not mine. I only issue STC's if requested to do so.

I have once seen a digital electric G meter. It was installed in a DA-42 I was flight testing, for the zero G testing we had to do. It seemed to work well, though I'm not sure if it would be "approved" for a certified installation. If it were to be installed in the electrical system, it would not be a major mod in and of itself for that - nor any other installation after a bus circuit breaker.

571 Appendix A says:

"(7) alter an electrical generation device, or the electrical distribution system between the generating source and either its primary distribution bus, or any other bus designated as an essential bus? "

[Instrument installation]
you see, in electrical terms, you have altered the electrical system between the generating source and the primary bus. That is without a doubt...whether your inspector knows that or not.
Anything you would install between the generator/alternator and the primary bus would be carrying the entire, or at least a very large, electrical load. The only "Instrument" I can think of which could carry this load would be an ammeter (shunt). That would be an alteration, and require "approved" data = STC, which is commonly available with the instrument for such installation. An MS/AN G meter is not that - it's not even electric - it's not connected to anything in the plane!

Otherwise, from the primary bus, there are generally circuit breakers, to which the various circuits are connected and thus protected. Anything you install to the "load" side of a circuit breaker is not between the generator and primary bus, it's after the primary bus, and therefore a "no" on the Appendix A "test" for major - in terms of electrical.

CID points to an absence of "acceptable" data, and I agree that AC43.13 makes no mention of G meters, so is not a source of acceptable data.

STD 571.06

" “acceptable data” - includes:

(a) drawings and methods recommended by the manufacturer of the aircraft, component, or appliance;

(b) Transport Canada advisory documents; and,

(c) advisory documents issued by foreign airworthiness authorities with whom Canada has entered into airworthiness agreements or memoranda of understanding such as current issues of Advisory Circular 43.13-1 and -2 issued by the FAA, Civil Aviation Information Publications (CAIPs) issued by the Civil Aviation Authority (CAA) of the United Kingdom, or Advisory Circular, Joint (ACJs) issued by the Joint Aviation Authority (JAA) or Acceptable Means of Compliance (AMC) issued by the European Aviation Safety Agency (EASA); and,
(amended 2009/12/01; previous version)

(d) drawings and methods found appropriate by a delegate in conformity with paragraph 4.2(o) and subsection 4.3(1) of the Aeronautics Act. (données acceptables)
(amended 2009/12/01; previous version) "

If (a) items are not available, (d) might be an option - the OP knows a delegate.

There's been some mis-information posted in this thread, so read with caution. The fact that something can be written in a post, does not make it authoritative. Always follow a lead back to the authoritative data to make your final determination.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#21 Post by hoptwoit » Thu Feb 04, 2016 10:44 pm

Good post. PilotDAR. Thanks for expanding on the electrical especially the between part.
I am not familiar with the aircraft. is a g meter required for aerobatics? I was only going by the optional part on the equipment list for the aircraft. I also agree it necessary to reference data of some kind for any install, minor or otherwise but at times with aviation is gets tough to arrive at black and white answers.

On that topic. I have seen the split bus bar relay replaced in many cessna with an avionics master switch. When reading through this topic that would be classed as major. Maybe there was an SB that covered it and I missed it. I don't want to derail the thread so if you feel this is off topic you can pm me if you like.
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#22 Post by torquey401 » Fri Feb 05, 2016 8:27 am

An informative resource - http://av8design.com/Topic_mods_home.htm

Relevant information that could be considered "approved data" - AC43.13-2B Chapter 11 "Adding or Relocating Instruments"

Good discussion!
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Re: G-METER question (non TSO) - can I swap 3" unit with a 2

#23 Post by North Shore » Sun Feb 07, 2016 3:10 am

This thread drifted into a discussion about electrical systems, which can be continued here: http://www.avcanada.ca/forums2/viewtopi ... 7&t=107450
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