Depends. Most of the early Aero Commanders, the Piper Aerostars and a few others have only one hydraulic pump. They are certified with this. In my case, it's even on the critical engine. This means that you can not raise gear if that engine fails. Yes, at SL you might eke out a slight climb if you do everything right and the other engine is feathered, but you wont meet IFR climb gradients by any means. Add high altitude to this and you won't climb at all.
So the right answer is - it depends on what you fly and what the circumstances are.
Last edited by AdamFrisch; 20th Dec 2012 at 13:38.