Rwy20 wrote:
But there are things like lightning protection which are not only dependent on the equipment, but rather the airframe itself. So if your AFM states a limitation to VFR, that is not necessarily an equipment issue!?
True.
So when the airframe is VFR only, you would install a GTN as VFR only as well, which also requires the placard GPS VFR only.
This depends, for example one some aircraft one must use overbraid over the coax cables with a GTN installation. In some of these aircraft this wasn’t required with excisting (factory standard) GPS, while the aircraft is IFR certified.
Sometimes these items come from the manufacture, like Garmin, requesting a second COM/NAV for example or in some cases a second GPS, while other manufacturers choose for different options. Then it also depend on when the aircraft and the avionics have been certified. HIRF / lightning requirements have changed over time, becoming more strict.
@lenthamen
“- pitot heat
- Alternate static
- Alternate Avionic Master
These are mandatory for IFR."
I partly disagree. The pitot heat is required by NCO.IDE.A.125, the alternate static source is optional (it is only a GM to NCO.IDE.A.125). The alternate avionic master doesn’t show up anywhere in NCO and therefore is either covered by the type certification nowadays or it is not required at all (this is most certainly the case for CAR3 certificated types).
An alternative avionics master switch may not turn up in Part NCO but the requirement to prevent a single point failure for loss of all radios is covered under AC23-1309 when certifying the basic aircraft or any subsequent modification. If the potential failure is identified as Hazardous then suitable mitigation has to be provided and this is where a back-up avionics master switch would be installed. One of the appendices to AC23-1309 describes loss of all Nav and Comm as the hazardous failure (only where radios are a mandatory item).
The UK CAA mandated the alternative avionics master prior to EASA under a requirement called Airworthiness Notice 84 ( now called CAP 747 GR18) and this still applies to UK Annex 11 aircraft.
There still are however planes with a single point of failure. For example, as I believe @bookworm will tell you, the TwinCom has a single switch which, if a wire comes off on the back, will cause a total loss of all electrics.
The “modern” avionics master switch override is just a switch which de-energises the avionics master relay, which is wired backwards so it is energised in the “avionics off” state.
And obviously any single alternator aircraft is the same. I guess the battery is then regarded as an adequate backup?
Very few twin alternator (SE or ME) planes have the proper system with dual buses and crossover switches. Even the lower-end Citations don’t have that, AFAIK. The one SEP which does is the Cessna TTX/400.
Peter wrote:
Aviathor makes an excellent point. The FARs make no provision for doing the VOR check in Europe, unless you use two VOR receivers. Personally I check against a GPS track and log it accordingly.
There is a good reason why this is not permitted in the US. Once a VOR is installed and aligned to the then current magnetic variation, it is rarely realigned as the variation drifts. There are many VOR’s that were installed in the 1960’s, more than 50 years ago, that have never been realigned to the current variation. Over the years, the variation has change in some cases 6 degrees. The radials have not moved in the mean time, so if you set the airway radial that is shown on the enroute chart, setting the value on the chart into the OBS will guide you over the same pile of dirt. But the track for a GPS will be using a current variation from its database. So in the example of the VOR located where the variation has drifted 6 degrees, the exact same radial is used, but the GPS will show a track difference of 6 degrees on a no wind day. That error is not compensated for in the comparison and easily exceeds the error budget, even when both the VOR and GPS are in perfect condition. If you are navigating towards a VOR on both the GPS and the VOR, the CDI for the GPS will use the old variation figure (called the VOR declination), but the track will use the current magnetic variation. So the GPS CDI will center at roughly the same point as VOR, but because the VOR is angular and the GPS is linear with a CDI full scale deflection in enroute mode of +/- 2 NM for a TSO C145/146 unit and +/- 5 NM for a TSO C129 unit, the error can be significant, particularly at low distances and high distances have the difficulty in centering the GPS CDI indication as the resolver is not adjustable with a fine enough resolution.
So bottom line, it is not a valid comparison. At least with a GNS530, there is a VOR radial display based off of the VOR signal that can be used without having to center the 530 CDI as the CDI should be checked as part of the power on self test on every flight for proper calibration of the OBS and CDI deflection.
Peter wrote:
Very few twin alternator (SE or ME) planes have the proper system with dual buses and crossover switches. Even the lower-end Citations don’t have that, AFAIK
Not sure about older Citations but certainly the Mustang and M2 have very good electrical redundancy.
There is a good reason why this is not permitted in the US. Once a VOR is installed and aligned to the then current magnetic variation, it is rarely realigned as the variation drifts.
AFAIK all the European ones are re-aligned continuously and probably they can do it remotely over a phone line – each VOR has a landline going to it.
I have never seen a variation of more than about 1 degree against a GPS.
There have been rumours posted in Europe that some well briefed ramp inspector in France had the “30 day VOR check” in his sheet. But I bet he didn’t know this level of detail, nor the bit about TSO 146 GPSs not needing the VOR checks, nor the VOR checks apparently not being needed for IFR unless VORs are actually used (I didn’t know that).
Peter wrote:
AFAIK all the European ones are re-aligned continuously and probably they can do it remotely over a phone line – each VOR has a landline going to it.
It may occur more frequently in the UK because of the amount of the movement of the variation. I checked a route between two VORs CLN Q295 to BPK and the error is 2 degrees along the 265 radial from CLN. That is bound to cause a lot of work and flight testing each time they are changed.
wigglyamp wrote:
The UK CAA mandated the alternative avionics master prior to EASA under a requirement called Airworthiness Notice 84 ( now called CAP 747 GR18) and this still applies to UK Annex 11 aircraft.
Do you also have to have an alternative main master, too? Given all the aircraft I’ve seen, the master switch is upstream of the avionics master (so turning off the main master will turn off all the avionics regardless of the state of the avionics master). Is the alternative avionics master simply wired in parallel with the avionics master?
Peter wrote:
There still are however planes with a single point of failure. For example, as I believe @bookworm will tell you, the TwinCom has a single switch which, if a wire comes off on the back, will cause a total loss of all electrics.
My total electrical failure story was actually in a PA28, 30 years ago, on my first night solo. The Twin Comanche has the usual alternative avionics master set-up.
