Groundspeed.  Speed over the ground, either closing speed to  the station or waypoint, or speed over the ground in whatever  direction the aircraft is going at the moment, depending upon  the navigation system used.  Height above landing (HAL).  The height above a designated  helicopter landing area used for helicopter instrument  approach procedures.  GS.  See glideslope.  GWPS.  See ground proximity warning system.  Gyroscopic precession.  An inherent quality of rotating bodies,  which causes an applied force to be manifested 90° in the  direction of rotation from the point where the force is applied.  HAA.  See height above airport.  HAL.  See height above landing.  HAT.  See height above touchdown elevation.  Hazardous attitudes.  Five aeronautical decision-making  attitudes that may contribute to poor pilot judgment: anti- authority, impulsivity, invulnerability, machismo, and  resignation.  Hazardous Inflight Weather Advisory Service (HIWAS).  An en route FSS service providing continuously updated  automated of hazardous weather within 150 nautical miles of  selected VORs, available only in the conterminous 48 states.  Head-up display (HUD).  A special type of flight viewing  screen that allows the pilot to watch the flight instruments  and other data while looking through the windshield of the  aircraft for other traffic, the approach lights, or the runway.  Heading.  The direction in which the nose of the aircraft is  pointing during flight.  Heading indicator.  An instrument which senses airplane  movement and displays heading based on a 360° azimuth,  with the final zero omitted. The heading indicator, also called  a directional gyro (DG), is fundamentally a mechanical  instrument designed to facilitate the use of the magnetic  compass. The heading indicator is not affected by the forces  that make the magnetic compass difficult to interpret.  Headwork.  Required to accomplish a conscious, rational  thought process when making decisions. Good decision- making involves risk identification and assessment,  information processing, and problem solving.  Height above airport (HAA).  The height of the MDA above  the published airport elevation.  HUD.  See head-up display.  HSI.  See horizontal situation indicator.  Hot start.  In gas turbine engines, a start which occurs with  normal engine rotation, but exhaust temperature exceeds  prescribed limits. This is usually caused by an excessively  rich mixture in the combustor. The fuel to the engine must  be terminated immediately to prevent engine damage.  Horsepower.  The term, originated by inventor James Watt,  means the amount of work a horse could do in one second.  One horsepower equals 550 foot-pounds per second, or  33,000 foot-pounds per minute.  Horizontal situation indicator (HSI).  A flight navigation  instrument that combines the heading indicator with a CDI,  in order to provide the pilot with better situational awareness  of location with respect to the courseline.  Homing.  Flying the aircraft on any heading required to keep  the needle pointing to the 0° relative bearing position.  Holding pattern.  A racetrack pattern, involving two turns  and two legs, used to keep an aircraft within a prescribed  airspace with respect to a geographic fix. A standard pattern  uses right turns; nonstandard patterns use left turns.  Holding.  A predetermined maneuver that keeps aircraft  within a specified airspace while awaiting further clearance  from ATC.  HIWAS.  See Hazardous Inflight Weather Advisory Service.  Histotoxic hypoxia.  The inability of cells to effectively use  oxygen. Plenty of oxygen is being transported to the cells  that need it, but they are unable to use it.  High performance aircraft.  An aircraft with an engine of  more than 200 horsepower.  Hg.  Abbreviation for mercury, from the Latin hydrargyrum.  HF.  High frequency.  Height above touchdown elevation (HAT).  The DA/DH or  MDA above the highest runway elevation in the touchdown  zone (first 3,000 feet of the runway).  H  G-14 