Inside the Cockpit – Altimeter

One of the more important instruments on the cockpit is the Altimeter.  The altimeter senses pressure changes and displays altitude in feet.  Just like I mentioned in prior posts, there are several different types of altitude.  The altimeter reads indicated altitude.  You’ll notice that the altimeter has three different hands or pointers similar to a clock.  The big hand/pointer displays 100s of feet.  The medium hand/pointer displays 1,000s of feet.   The smallest hand (in this case just a triangle with a white line) displays 10,000s feet.  For example, the altimeter reading below is just under 800 feet (most likely above sea level)

  Since the altimeter reads pressure, the outside barometric pressure is important.  The standard atmosphere, or the atmospheric pressure at sea level is 29.92 inches on mercury (Hg.).  Since barometric pressure changes, the pilot needs to adjust the setting prior to take off.  When checking the weather, visibility and other factors, the pilot should note the barometric pressure and adjust the altimeter with the small knob in the lower left corner.  Most airports are a few hundred feet above mean sea level (MSL).  For example, the Smyrna airport is about 550 feet above MSL.  The highest airport in the U.S. is Leadville Airport is over 9,900 feet above MSL, and that is before you ever leave the runway.



The altimeter will not display how high you are from the ground (absolute altitude), but will display the distance above mean sea level after it has been adjusted.  For each one inch of change in Hg the altimeter will be off by 1,000 feet.  While traveling , the pilot will need to check the barometric pressure.