Inverters

[TheFinn88 13]

Hi.

I've some questions about the inverters.The POH seems to describe another inverter-system to the one installed in the PMDG DC-6. In the POH are three switches mentioned for operating the captains, first officers and engine instruments on either the lower or upper inverter. But the DC-6 in the sim seems to have three inverters (No.1, No.2 and one for the engine instruments), where both No.1 and No.2 power both flight instruments sides (capt & f/o) and the third one only for the engine instruments.

When turning only one of either No.1 or No.2 inverters on, both flight instruments power failure lights extinguish. I assume that the flight instruments of both sides (capt & f/o) are wired to both inverters, so in case of an inverter failure the flight instruments receive AC-power from the remaining functioning inverter. Is that correct?

Second question is; what is the stanb-by position of all three inverter switches for? As I've seen so far, there is no difference between the off or stand-by position (in stand-by the systems also have no AC-power).

Third question is; what do I do in the case of a engine instruments inverter failure? Is there a way to power the engine instruments from another AC-source?

I use the P3Dv4 Version.

Regards.

[jimmyfingers 2]

Great questions!  Thanks for posting.  I'll be interested in the answers.

[downscc 1,327]

There is a system description for the electrical system on page 94 in the POH but it is an original Douglas publication and the PMDG product is a copy of an actual aircraft that had (as most have had) modifications here and there.  An electrical system is most prone to modification because components become obsolete and irreplaceable. For example, the inverters back then were DC motors driving an AC alternator, which was about the only way they had of converting DC to AC.  If one of those fails today you either rebuild it or replace it with a  solid state inverter.

I agree, the assumption that flight instruments are powered by either CPT or FO inverter appears valid and is basically what the POH says "either of which is capable of supplying the entire .. AC load for instruments and electronic equipment operation."  It appears this aircraft was modified removing the upper and lower inverters and replacing with  No.1 and No.2 inverters.  It just occurred to me that Douglas may have built it this way.

The standby position for inverters might be a state where the DC motor is powered and up to speed but the alternator field coil is not energized. This might be an alternative to applying power to an inverter and then waiting a period of time before applying loads to allow for spin-up.

I notice the POH has an engine selector switch to select either upper or lower inverter for source of 26VAC power (for engine instruments).  Our DC6 has a third inverter for engine instruments and does not have an emergency inverter which would have backed up either upper or lower.  The schematic on pg 100 lists the engine instruments that rely on 26VAC but I find that BMEP works without power.  What we actually loose with 26VAC failure are the fuel flow/pressure and oil pressure, flap position and I didn't check the others. We still get BMEP, which could be a bug in the PMDG simulation.  Of course RPM and MP are mechanical gauges so they still work.

Good question.  This took longer than most to answer.  I think you have a valid issue with the BMEP instrument working with the engine instrument inverter turned off, and recommend you submit that on a trouble ticket to PMDG.  That could result in them checking with their technical source on this specific aircraft.

[TheFinn88 13]

12 hours ago, downscc said:

Good question.  This took longer than most to answer.  I think you have a valid issue with the BMEP instrument working with the engine instrument inverter turned off, and recommend you submit that on a trouble ticket to PMDG.  That could result in them checking with their technical source on this specific aircraft.

Thanks for the extensive answers. Honestly I didn't realize that the BMEP instruments still work without electrical power. I'm gonna try to recreate that and then submit a trouble ticket to PMDG. I will also have a look at all the other engine instruments (apart from MAP and RPM which are mechanical gauges).

Another thing I noticed is, that the water pressure instruments (ADI) always show some pressure (about 10 on the scale), even when there is no electrical power at all. Are these also mechanical gauges, which show a pressure leftover in the system? Or is this a bug? Or even an quirk of the real DC-6 on which the PMDG is based on?

Regards

[downscc 1,327]

10 hours ago, TheFinn88 said:

Another thing I noticed is, that the water pressure instruments (ADI) always show some pressure (about 10 on the scale), even when there is no electrical power at all. Are these also mechanical gauges, which show a pressure leftover in the system? Or is this a bug? Or even an quirk of the real DC-6 on which the PMDG is based on?

You should have checked the POH.  See para 13.4 pg 179. That pressure indication is normal.

[TheFinn88 13]

2 hours ago, downscc said:

You should have checked the POH.  See para 13.4 pg 179. That pressure indication is normal.

Ooops, missed that one. And its working fine, they show 0 pressure with the engine instrument inverter is turned off. And when the inverter is turned on, they show about 12 on the scale. As it is described in the POH.

But for the BMEP indicators, they indeed still work even when the engine insturment is turned off (also when all 3 inverters are turned off). Also all the temperature gauges (oil-temp, carb-temp, cyl.head-temp) keep working with the eng. instrument inverter turned off. Wonder if they are electrical or mechanical gauges...

[downscc 1,327]

29 minutes ago, TheFinn88 said:

Also all the temperature gauges (oil-temp, carb-temp, cyl.head-temp) keep working with the eng. instrument inverter turned off. Wonder if they are electrical or mechanical gauges...

Temperature instruments are generally "mechanical" in a sense, in that the temperature sensor is a thermocouple that has an output voltage in the mV range that varies with temperature so there is no need for external power. Thermocouples have been around a long time and are inexpensive, reliable and for a given range they are also accurate.

[TheFinn88 13]

The ticket regarding the BMEP-instruments working without the enginge instrument inverter turned on is opened. And regarding the temperature-instruments you are right Dan. According to the POH (Page 99 / Figure 46 — A-C Power Supply System) following engine instruments are electrically driven:

• Fuel Flow Indicator
• Fuel Pressure Indicator
• Oil Quantity Indicator
• Oil Pressure Indicator
• BMEP Indicator
• Heater-Fuel Pressure Indicator
• Wing Flap Position Indicator
• Cabin Supercharger Gear Box
• Oil Pressure Indicator

So from my point of view, apart from the BMEP-instruments, everything is working as it should. The waterpressure-indicators for the ADI aren't mentioned here though. But they also stop indicating when the engine instrument inverter is turned off.

[jimmyfingers 2]

Thanks for drilling down on this thread.  This subject is an excellent example of the depth of detail and effort PMDG put into this project.  I think it is a real testament to the team to be able create such a high fidelity model with a legacy simulator.  Well done, y'all!

[TheFinn88 13]

I've got an answer on my ticket regarding the BMEP gauges. The plane is fine, the POH is wrong. According to the dev's, the BMEP gauges are "engine driven", so they work without any electrical power at all, exactly like what I've seen in the sim. The POH will be corrected on a future update.