I cannot stress enough the importance of having control of the bleed valves prior to flight. If you cannot control the bleed valves in flight you can end up causing a lot of damage to the aircraft.
Case in point. One winter afternoon we departed Aspen Co. (KASE) in a Sabre 65. As it was a cold day we did a bleeds on takeoff, the normal configuration for takeoff. In Aspen when the ambient temperature was above 10c we would do a bleeds off takeoff for performance, however that day it was -05c so we had more than adequate performance with the bleeds on.
Going through FL180 the automatic cabin temperature controller failed and called for full hot air. The first thing I noticed, before we realized that the controller failed, was a sudden increase in the noise of the air coming out of the ducts, then a rapid increase of the air temperature coming out of the ducts and then the smell of something getting too hot. I looked at the valve position indicator and it was nearly at the full open position. Then we got a "HOT CABIN AIR" warning on the annunciator panel. I turned the cabin thermostat down to full cold, no change. Then I switched the cabin temperature controller to manual and toggled the switch to full cold, no change.
The air in the cabin and now the cockpit was becoming extremely hot. By throttle movement we determined that it was the right engine bleed valve that had failed. At that point we decided that we were going to have to shut down the right engine or risk serious consequences. Before we shut the engine down I decided to try one more thing. According to valve position indicator the valve was not full open, just about 90% open. So I toggled the valve to hot, it moved and I immediately switch the direction of the valve to cold and it moved to the full cold position. The noise stopped, the cabin and cockpit air cooled down and the "HOT CABIN AIR" light went out. We then continued on the flight back to our home base.
Writing this story took longer than the actual incident. We were lucky as I was able to regain control of the bleed valve before any damage was done to the duct works. On tear-down of the valves we found all kinds of debris inside the valves that had caused the right valve to jam. I had the old valves replaced and told the director of maintenance to modify our inspections to remove and clean the valves more frequently. We notified Sabreliner of the incident and the change in our maintenance procedures. Shortly after this Sabreliner sent out a recommendation to inspect the valves more frequently.
We had most likely brought this problem on ourselves, as we only checked the valves once a day prior to the first flight and, this is my fault as I was the Chief Pilot, we did not close the valves on every shutdown as called for. Now on the Falcon 50 and 900 aircraft all takeoffs are bleeds off, but that is a automatic function, as N1 passes around 60% the bleeds close and open shorty after you are airborne.