Graduated collar for altimeter 4FGH20 or 4FGH10.
On cross country flights the altimeter is, in general, set to the field elevation value of departure airport. The colesman-window shows the air pressure reduced to sea level (QNH). During the flight the altimeter indicates altitude above sea level. In many cases the altitude relative to a certain terrain feature is of interest, in addition to the altitude above sea level. When on final approach the altitude above the airfield must be known. For this reason the arrow of the graduated collar is moved to indicate the airfield altitude. Now the remaining height to landing spot altitude can be read from the altitude index on the collar at all times (indication equals QFE). It can also be used to ensure safe overflight of ground obstacles (mountains) or control zones because the altitude difference can be read directly from the collar after appropriate adjustment. Usage has proven very useful to support final approach calculations on cross country flights.
