My understanding is that the “C” model still has a vacumn driven Artificial Horizon. Why didn’t Cirrus go to an Electric Aritificial Horizon and remove the vacumn pump and the back up vacumn system to save weight? I think the reliablity of the electric gyros is much higher than the vacumn driven gyros. Does anyone know the cost/weight differences to do this??? Walt what do you think? You noted that you have already lost a vacumn pump!!
Bernie wrote:
Why didn’t Cirrus go to an Electric Aritificial Horizon and remove the vacumn pump and the back up vacumn system to save weight?
The HSI is also vacuum operated. I have asked Cirrus about the Sandel Electronic HSI, and they are considering it. I would imagine if this was fitted, then it would make a lot of sense to change the AH to electric too. The C model already has dual alternators.
Walt what do you think? You noted that you have already lost a vacumn pump!!
Since you asked … First, I am too cheap to think about the ‘C’ model. Afterall we only have the ‘cheap A model’. I read Dave Katz’s comments and he is right on target. Having dual systems (electric and vacuum) has some advantages should one fail. The point that Dave makes is extremely valid about having an electric rate-based autopilot that you can use should you lose vacuum. Moreover, Cirrus in its current design provides a slick backup electric vacuum pump that automatically kicks in should the engine driven vacuum pump fail. Personal experience with watching the engine driven vac fail was nothing more than watching the ‘idiot light’ come on on the panel … no big deal. It simply said “the plane needs a new engine driven vacuum pump”.
In the for what it is worth column, no matter how much you spend you cannot eliminate all risk, and I am not convinced that the ‘B’ or ‘C’ packages eliminate enough risk given the extra costs, but they do give you more “toys”. Good luck.
Bernie wrote:
Why didn’t Cirrus go to an Electric Aritificial Horizon and remove the vacumn pump and the back up vacumn system to save weight?
The HSI is also vacuum operated. I have asked Cirrus about the Sandel Electronic HSI, and they are considering it. I would imagine if this was fitted, then it would make a lot of sense to change the AH to electric too. The C model already has dual alternators.
The HSI in the B model is vacuum-operated, but the HSI in the C model is electric. The only vacuum instrument in the C model is the AI.
A vacuum AI means that if you end up with a total electrical failure (for reasons other than losing one alternator, since you have a second, but there are other single points of failure) then at least you have a mag compass, an AI, and the pitot/static instruments.
If you have a total vacuum failure, you’ll still have a fully functioning autopilot for all modes, so if you cop a clue and engage the autopilot instead of following the leaning AI into the ground you will be in good shape.
Note that the Sandel requires external gyros and slaving, so that stuff has to go somewhere as well.
Dave wrote:
The HSI in the B model is vacuum-operated, but the HSI in the C model is electric.
Oh, you’re right. I didn’t read closely enough 
Any idea why the C model has the electric HSI? I would not have thought there was much difference in price.
Note that the Sandel requires external gyros and slaving, so that stuff has to go somewhere as well.
The NSD has an external slave unit anyway. The Sandel gyro is external, too, but that shouldn’t be a big problem.