Sectors depicted on approach charts must be at least 90° in scope. These altitudes are for emergency use only and do not necessarily assure acceptable navigational signal coverage.

2.  
Emergency Safe Altitudes: Altitudes depicted on approach charts which provide at least 1,000 feet of obstacle clearance in nonmountainous areas and 2,000 feet in designated mountainous areas within a 100-mile radius of the navigation facility upon which the procedure is based and normally used only in military procedures.

MSAW (minimum safe altitude warning). A function of the ARTS III computer that aids the controller by alerting him when a tracked Mode C-equipped aircraft is below, or is predicted by the computer to go below, a predetermined minimum safe altitude.
MSB (most significant bit). The bit in a binary number that has the highest place value. The MSB is the bit on the extreme left.
MSD (most significant digit). The digit in a decimal number that has the highest place value. The MSD is the digit on the extreme left.
MSDS (material safety data sheets). Documents relating to hazardous materials that contain information on health precautions, flammability, ventilation requirements, and information for health professionals in case of an accident. MSDS are furnished with all hazardous materials and are required by law to be available to all personnel working with the material.