Standby instruments

[Flycloud]

Are the standby instruments of the DA42 built the same as any steam gauge diamonds? Example, the altimeter has the aneroid wafer, while the airspeed indicator has the diaphragm connected to the pitot tube, and pitot static to its casing?

Same with attitude indicator and the gyro, except that it is electrical?

Regards.

[erchegyia]

Sorry, I cannot answer the question, but I recently flew on an FNTP II simulator and had a total electrical failure. After turning the emergency switch on, the standby instruments took ~15 seconds to 'boot'. One can claim that this is an improbable scenario as you usually have a main battery for a while before you need to go for the emergency switch (do they reboot then as the power source changes?). However, still... if this happens in IMC you can be easily inverted by the time you get the first attitude indication. If this works the same way in the real aircraft, how could it pass the certification?!

[Rich]

Sorry, I cannot answer the question, but I recently flew on an FNTP II simulator and had a total electrical failure. After turning the emergency switch on, the standby instruments took ~15 seconds to 'boot'. One can claim that this is an improbable scenario as you usually have a main battery for a while before you need to go for the emergency switch (do they reboot then as the power source changes?). However, still... if this happens in IMC you can be easily inverted by the time you get the first attitude indication. If this works the same way in the real aircraft, how could it pass the certification?!

The airspeed indicator, compass and altimeter are mechanical and do not rely on electrical power to operate. The attitude indicator as originally supplied is a "stupid" spinning gyro that still works for awhile after losing power due to its inertia. It no more "boots up" than turning on a light. If allowed to spin down completely it takes a couple of minutes to come to full RPM. But this wouldn't reflect a real power loss situation in flight.

The term "standby" can be interpreted inaccurately. These fully operate in flight in parallel to the G1000 (or whatever). If the simulator you're using doesn't emulate the actual operation, I wouldn't be surprised.

[erchegyia]

The airspeed indicator, compass and altimeter are mechanical and do not rely on electrical power to operate. The attitude indicator as originally supplied is a "stupid" spinning gyro that still works for awhile after losing power due to its inertia. It no more "boots up" than turning on a light. If allowed to spin down completely it takes a couple of minutes to come to full RPM. But this wouldn't reflect a real power loss situation in flight.

The 'analogue' ones of course... But quite some are equipped with something like this: MidContinent SAM - https://www.mcico.com/flysam

[chili4way]

From the MidContinent website:

"SAVE THE DAY
SAM’s rechargeable, internal battery saves the day and keeps you flying in the event of electrical failure. You’ll have one hour of emergency power and the critical time you need to land safely."

[Rich]

From the MidContinent website:

"SAVE THE DAY
SAM’s rechargeable, internal battery saves the day and keeps you flying in the event of electrical failure. You’ll have one hour of emergency power and the critical time you need to land safely."

Right. I haven't tested any, but these types of devices generally have their own backup batteries and are supposed to seamlessly switch when necessary.

[chili4way]

In any of the Diamond diesels, the limitation to continued flight after complete primary battery exhaustion is the ECU backup battery. That's about 30 minutes under normal conditions, less time than the duration of the backup instrument batteries. With magneto ignition, it depends on fuel vs. the backup instrument batteries.

[erchegyia]

Right. I haven't tested any, but these types of devices generally have their own backup batteries and are supposed to seamlessly switch when necessary.

Ok, then it does work different in the sim

I only know the DA40 with analogue backup gauges by heart: the emergency switch connects the single-use battery which powers the standby instruments and the flood light. As you explained, this is perfectly fine because the gyro has quite some intertia.

If the electric instruments have built-in battery switching seamlessly (makes sense!) what does the emergency switch do?

[Steve]

In mine, the Emergency Switch operates the Floodlight only. The electronic AI has its own backup battery, plus the 45 volts supplied by the Emergency battery is outside of its operating voltage specifications.

[chili4way]

Nit clarification: The "horizon emergency battery" is 30V (10 lithium manganese batteries, 3 V, 1,300 mAh), so about the same voltage as the 28V alternator that usually powers the electroluminescent flood light and backup AI or SAM. The supply voltage spec for the MidContinent SAM is 10V-32VDC.

According to the AMM wiring diagrams (D44-9234-10-01X01), the SAM is powered by the emergency battery when that switch is turned on. So, the SAM will nominally run much longer than one hour.

This made me wonder why the SAM doesn't stay powered up using its internal battery after a normal shutdown. According to the Pilot Guide, It uses the pitot/static inputs to determine that the airplane isn't flying (no airspeed) and turns itself off.