Flight Mechanic

Aircraft Mechanic School Study Supplement for Future Aviation Maintenance Technicians




You are here: Home / Basic Aviation Maintenance / Aircraft Weight and Balance / Weight and Balance Terminology – Arm
Regretfully, Flight-Mechanic will be turning out the lights after fifteen years. Google, in its infinite wisdom, has chosen to remove Flight-Mechanic from its search results (the claim is that the content on this site is spam). We appealed their decision to shut us down, to no avail.

Unfortunately, since Google has a monopoly over internet search, this means that traffic levels on the site will drop to a level that makes maintaining it uneconomic. As time progresses, we will no longer be able to maintain the server space and will remove the site from the internet. Thanks to all who have supported us over the years.

To fight back against the monopolistic practices of companies like Google, we recommend using a competing search engine such as Brave (you get the added benefit of not being spied on by Big Tech) and advocating for anti-trust legislation from your representatives.

Weight and Balance Terminology – Arm

Filed Under: Aircraft Weight and Balance

The arm is the horizontal distance that a part of the aircraft or a piece of equipment is located from the datum. The arm’s distance is always given or measured in inches, and, except for a location which might be exactly on the datum, it is preceded by the algebraic sign for positive (+) or negative (−). The positive sign indicates an item is located aft of the datum and the negative sign indicates an item is located forward of the datum. If the manufacturer chooses a datum that is at the most forward location on an aircraft (or some distance forward of the aircraft), all the arms will be positive numbers. Location of the datum at any other point on the aircraft will result in some arms being positive numbers, or aft of the datum, and some arms being negative numbers, or forward of the datum. Figure 4-1 shows an aircraft where the datum is the leading edge of the wing. For this aircraft, any item (fuel, seat, radio, and so forth) located forward of the wing leading edge will have a negative arm, and any item located aft of the wing leading edge will have a positive arm. If an item was located exactly at the wing leading edge, its arm would be zero, and mathematically it would not matter whether its arm was considered to be positive or negative.

Figure 4-1. Datum location and its effect on positive and negative arms.
Figure 4-1. Datum location and its effect on positive and negative arms.

The arm of each item is usually included in parentheses immediately after the item’s name or weight in the Aircraft Specifications, Type Certificate Data Sheet, or equipment list for the aircraft. In a Type Certificate Data Sheet, for example, the fuel quantity might be identified as 150 gallons (gal) (+138) and the nose baggage limit as 200 pounds (lb) (−55). These numbers indicate that the fuel is located 138″ aft of the datum and the nose baggage is located 55″ forward of the datum. If the arm for a particular piece of equipment is not known, its exact location must be accurately measured. When the arm for a piece of equipment is being determined, the measurement is taken from the datum to the piece of equipment’s own center of gravity.

Flight Mechanic Recommends

Rod Machado's Private Pilot Handbook -Flight Literacy recommends Rod Machado's products because he takes what is normally dry and tedious and transforms it with his characteristic humor, helping to keep you engaged and to retain the information longer. (see all of Rod Machado's Products).
   
-->