Altimeter shows 100 foot drop when power is added. Is this a leak in the static system or broken altimeter and how long will the fix take? My instructor and I were preparing for my Private Pilot Check ride today when we noticed that the analog altimeter would suddenly lose 100 feet when we added power.  The only way I can think to explain this is that there could be a leak in the static system that caused the altimeter to read the cabin pressure instead of outside static pressure.  We had the cabin vents open so that when the prop blew in more air it probably caused the cabin pressure to rise slightly causing the altimeter to read a lower altitude.  However, we only noticed an error with the altimeter and not with the airspeed or VSI.  At one point I was in a climb and the altimeter was showing my altitude decreasing and the VSI was showing an altitude increase.  If the problem was a leak in the static system you would think that the airspeed and VSI would be affected as well.  I did call the mechanic about this and he decided to take the airplane out of service.

My question is could this problem be isolated to just the altimeter and not the entire static system.  Also, what would the typical downtime be to fix this issue?  I am supposed to take my check ride Sunday but it looks like that may not happen if this would take awhile to fix.  

Note:  This airplane does not have the option of using an alternative static source so no it is not due to the alternate static source being selected.
The mechanic will do a static system leak check.  This is done by applying a sealed off suction source to the static port, then applying an inch of suction, causing the altimeter to rise a thousand feet, then holding that for one minute (I do it using a big syringe connected to the blanked off static port on my homebuilt).  The altimeter is supposed to not move more than 100 feet in 1 minute (that's the rather generous Canadian requirement - not sure of the FAA but I assume it's similar)

If there are no obvious static leaks, and the static port itself seems normal, the mech will probably swap out the altimeter and see what happens.  Or, swap out the altimeter first and then try it out.

The leak check will take about 30 minutes, and the altimeter swap-out a couple hours.  Shouldn't take more than a morning to resolve.