Repair station. A facility where specified aircraft and  their parts may be inspected, repaired, altered, modified,  or maintained. FAA approval is issued to a facility upon  qualifications specified by the local FSDO. Positive control.  According to Title 14 of the Code of Federal  Regulations, control of all air traffic, within designated  airspace, by air traffic control. Preflight.  All preparations, including gathering information,  assembly, and inspection performed by the pilot before  flight. Pressure gradient.  In weather, the difference between high  and low pressure areas. Wind speed is directly proportional  to the pressure gradient. Repairman certificate. An FAA certificate issued to a  person who is employed by a repair station or air carrier as a  specialist in some form of aircraft maintenance. A repairman  certificate is also issued to an eligible person who is the  primary builder of an experimental aircraft, to which the  privileges of the certificate are applicable. Required equipment. Equipment that must be aboard  an aircraft, as required either by the FAA or balloon  manufacturer, to maintain airworthiness. Pressure relief valve.  A device in a propane tank designed to  release excess pressure—which may be caused by overfilling,  overheating, or excessive nitrogen pressurization—to prevent  tank rupture. Preventive maintenance. Simple or minor preservation  operations and the replacement of small standard parts not  involving complex assembly operations. Restricted area. Airspace of defined dimensions within  which the flight of aircraft is restricted in accordance with  certain conditions. Return to service.  A certificated mechanic or authorized  inspector must approve an aircraft for return to service after  it has been inspected, repaired, or altered. In addition, an  aircraft that has been modified must be test flown by an  appropriately certificated pilot before return to service. Prohibited area.  According to Title 14 of the Code of Federal  Regulations, designated airspace within which the flight of  aircraft is prohibited. Ridge.  In weather, an elongated area of high pressure with  no rotative motion. Propane.  A colorless and odorless gas. Ethyl mercaptan is  added to propane to give it a detectable odor. Propane weighs  4.2 pounds per gallon at 60 °Fahrenheit. Practical Test Standard (PTS). Book containing areas  of knowledge and skill that a person must demonstrate  competency in for the issuance of pilot certificates or  ratings. Rip panel.  A deflation panel, usually circular or triangular,  at the top of a balloon envelope, which may be opened by  pulling a line in the basket to allow hot air or gas to escape,  and the envelope to deflate. Pull test.  A strength test in which a section of envelope fabric  is pulled to a definite pound measurement to determine if it  meets the certification requirements for airworthiness. Rotor. May be found embedded in mountain waves.  Formation usually occurs where wind speeds change in a  wave, or where friction slows the wind near the ground. See  Figure 4-22  of this handbook for a graphical representation  of rotors.  Pyrometer.  An instrument used to measure air temperature  inside the top of a balloon envelope. Rapid descent.  A relatively fast loss of altitude. A subjective  term, but usually meant to describe a descent of more than  500 fpm. Sectional chart.  Published on a routine basis, these charts  are similar to automobile road maps, and provide useful  information regarding airspace, reference points, tower  frequencies, etc., to a balloon pilot. They are generally not  very helpful for navigation, as the scale, 1:500,000, is too  small to be of use to the balloon pilot. Rating. According to Title 14 of the Code of Federal  Regulations, a statement that, as part of a pilot certificate,  sets forth special conditions, privileges, or limitations. Red line.  Refers to a line which activates the deflation panel  of a balloon, or the maximum envelope temperature allowed,  or the maximum on a gauge. SIGMET.  Significant Meteorological Information. Rotator vent.  See Turning Vent. G-7