They have been used in lots of planes
This article suggests that failures are most likely when access to the two securing nuts is poor. That in turn points to incompetent installations…
I read somewhere that Socata used this variant because there wasn’t enough room for the two separate mags, but the following proves that untrue: One TB20 owner was told by his FSDO that he can fit any engine into his TB20 so long as it shares the same TC with his engine. That is of course completely wrong, but he did it and put in the dual mag version. He has since been very reluctant to discuss it, unsurprisingly 
However clearly he cannot be prosecuted by the FAA.
Also the single shaft mag engine is about $1000 cheaper.
Apparently, FAA guidance excludes this type of change from requiring an STC. From the FAA Field Approval guidance (order 8900.1 vol4 chapter 9):
“Substituting an engine or propeller (such as replacing a reciprocating engine with a turbine engine) – STC”
“Substituting or altering a reciprocating engine such that the net result is an increase of more than 10 percent greater horsepower – STC”
So providing the engine has the same power it should be possible to do it via the 337 + Field Approval or 337 + DER 8110 routes.
Also very interesting that you could increase the power up to 10%. That is a LOT of extra power.
Peter wrote:
This article suggests that failures are most likely when access to the two securing nuts is poor. That in turn points to incompetent installations…
The fasteners were changed at some point, the original designs could crack the housing.
Peter wrote:
Also the single shaft mag engine is about $1000 cheaper.
Why are you asking for the reason when you just mentioned the one and only reason?
At least in the TR182, they needed one cam drive for the turbo oil scavenger pump so in addition to saving money on the mag, they also saved the cost/space for an adapter plate that turns one cam drive into two.
Alot of other planes that came with “D” Lycomings : Cessna 177RG, Cessna 172M, Partenavia P68, to name a few .
Peter wrote:
same TC with his engine. That is of course completely wrong, but he did it and put in the dual mag version.
I don’t follow that at all.
The engine model type is either listed in the aircraft’s TCDS or it isn’t.
Have no idea what you mean above.
Of course only the engine listed in the TCDS can be installed in the airframe.
Just because one Lyco TC lists say 5 engines doesn’t change anything.
sorry, what is the TCDS ? TC and STC I know, but this one ?
Peter wrote:
Just because one Lyco TC lists say 5 engines doesn’t change anything.
You mean that 5 variants of the same basic engine are listed on that engine’s TC ?
That being the case, its is totally irrelevant to the engine type listed on an aircrafts’ TCDS.
The Cessna 177RG’s TCDS lists both the Lyco IO-36 “D” and non “D” engine types so an owner/operator can change with nothing more than a log-book entry.
A TCDS is created by the certification authority (FAA, EASA, UK CAA, DGAC, etc) when they certify an aircraft.
The TCDS contains requirements on mandatory-installed equipment. It contains info on what bits may be installed. Importantly, anything listed in it can be installed as a pre-approved modification (not sure if that is actually true under EASA who like to examine everything if they get half a chance) i.e. just with a logbook entry.
Point of note, a ‘D’ suffix on a Lycoming model does not mean it definitely has a dual mag fitted – only if the ‘D’ is at the end of the model designator:
O-320-D2J – Cessna 172M is not a dual mag
O-320-D3G – Piper PA28-161 Warrior II is not a dual mag
O-320-H2AD – Cessna 172N is a dual mag
O-540-J3A5D – Piper PA28-236 Dakota is a dual mag
IO-540-K1G5D – Piper PA32R-301 Saratoga is a dual mag
Other types that use it include the Beech Duchess and the Rockwell 114. I’m not convinced that the Partenavia P.68 has dual mags fitted (Lycoming IO-360-A1B and -A1B6 by my understanding, but I stand to be corrected).
As an edit to my post above (seeing as my ‘edit’ option appears to have disappeared) and after a memory update:
The C172M has an O-320-E2D (150hp) but isn’t a dual mag, the -D2J (160hp) was in the C172P and still not a dual mag.
The dual mag designator is a ‘D’ in the 4th or 5th position in the model designator – still the last suffix.
