Modern INS systems are known as IRS. They are completely  solid-state units with no moving parts. Three-ring, laser  gyros replace the mechanical gyros in the older INS  platform systems. This eliminates precession and other  mechanical gyro shortcomings. The use of three solid-state  accelerometers, one for each plane of movement, also  increases accuracy. The accelerometer and gyro output  are input to the computer for continuous calculation of the  aircraft’s position.  inspect, maintain, and troubleshoot these ever increasingly  complicated electronic devices and systems. It is imperative to  follow all equipment and airframe manufacturers’ instruction  when dealing with an aircraft’s avionics.  The most modern IRS integrate is the satellite GPS. The GPS  is extremely accurate in itself. When combined with IRS, it  creates one of the most accurate navigation systems available.  The GPS is used to initialize the IRS so the pilot no longer  needs to do so. GPS also feeds data into the IRS computer to  be used for error correction. Occasional service interruptions  and altitude inaccuracies of the GPS system pose no  problem for IRS/GPS. The IRS functions continuously and  is completely self-contained within the IRS unit. Should the  GPS falter, the IRS portion of the system continues without  it. The latest electronic technology has reduced the size and  weight of INS/IRS avionics units significantly.  Figure 11-156 shows a modern micro-IRS unit that measures approximately  6-inches on each side. The revolution to GPS navigation and the pace of modern  electronic development results in many aircraft owner  operators upgrading flight decks with new avionics. The  aircraft technician must only perform airworthy installations.  The avionics equipment to be installed must be a TSO’d  device that is approved for installation in the aircraft in  question. The addition of a new piece of avionics equipment  and/or its antenna is a minor alteration if previously approved  by the airframe manufacturer. A certificated airframe  technician is qualified to perform the installation and return  the aircraft to service. The addition of new avionics not on  the aircraft’s approved equipment list is considered a major  alteration and requires an FAA Form 337 to be completed.  A technician with an inspection authorization is required to  complete the approval for return to service following a major  alteration and to sign the corresponding approval for return  to service block on Form 337. Most new avionics installations are approved and performed  under an STC. The equipment manufacturer supplies a list  of aircraft on which the equipment has been approved for  installation. The STC includes thorough installation and  maintenance instructions which the technician must follow.  Regardless, if not on the aircraft’s original equipment list,  the STC installation is considered a major alteration and an  FAA Form 337 must be filed. The STC is referenced as the  required approved data. Most of the avionics equipment discussed in this chapter is only  repairable by the manufacturer or FAA-certified repair stations  that are licensed to perform specific work. The airframe  technician; however, must competently remove, install,  Occasionally, an owner/operator or technician wishes to install  an electronic device in an aircraft that has no STC for the  model aircraft in question. A field approval and a Form 337  must be filed on which it must be shown that the installation  will be performed in accordance with approved data. There are many factors which the technician must consider  prior to altering an aircraft by the addition of avionics  equipment. These factors include the space available, the size  and weight of the equipment, and previously accomplished  alterations. The power consumption of the added equipment  must be considered to calculate and determine the maximum  continuous electrical load on the aircraft’s electrical system.  Each installation should also be planned to allow easy access  for inspection, maintenance, and exchange of units. The installation of avionics equipment is partially mechanical,  involving sheet metal work to mount units, racks, antennas,  and controls. Routing of the interconnecting wires, cables,  11-72 Figure 11-156.  modern micro-IRS with built-in GPS. Considerations Installation of Communication and  Navigation Equipment Approval of New Avionics Equipment  Installations CA UTION CA UTION