Glossary - Letter O

If you wish to find a term by name, please click the relevant letter below to be taken to a list.


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OAT |abbreviation|  1. operational air traffic  2. outside air temperature


obey |verb|  1. to carry out or comply with a command / Pilots must obey landing instructions. /  2. to follow a physical law / Winds obey Buys Ballot’s Law. /


OBI |abbreviation| omni-bearing indicator


object |noun|  1. something that you can touch and see and that has a particular form and dimensions / Any given object will collect more ice when traveling at high speed than at low speed. /  2. intention or aim / The object of the briefing is to inform all aircrew of the new procedures. / - |verb| to raise or voice opposition / Staff objected to the introduction of longer working hours. /

oblique exit / turn-off |noun| an exits designed to facilitate aircraft vacating the runway at speed, hence the term ‘high-speed turnoff’, and performing a rolling start for take-off

oblong |adjective| rectangular |noun| a rectangle


OBS |abbreviation| omni-bearing selector

obscuration |noun| a decreased visibility caused by fog, smoke, sandstorm etc.

obscure |adjective| not clearly understood - the explanation was obscure > the explanation was difficult to understand because it wasn’t clear - |verb| to make difficult to see / Deposits of ice crystals on the windscreen will obscure vision. /


obscured |adjective| - sky obscured > a meteorological term to mean that fog or mist prevents sight of the sky


observation |noun| careful watching / The type of cloud is established by observation and comparison with cloud photographs. /


observe |verb| to watch carefully / Local wave action can be observed from a height of 200 feet. Wing deflection can be observed from the passenger cabin. /


observer |noun| a person working in a meteorological station who assesses weather conditions by visual means / Meteorological visibility is the greatest horizontal distance at which objects can be seen and recognized by an observer on the ground with normal eyesight and under conditions of normal daylight illumination. /


obstacle |noun| something which blocks a path or prevents progress / Low frequency transmissions can penetrate obstacles such as mountains. Knowing the heights of obstacles en route, it must be ensured that in the event of an emergency, the flight may be continued in safety. /


obstacle clearance |noun| the fact of being at a sufficient height to be able to fly over any obstacles in the area


obstruct |verb| to block a path or to prevent the progress of something / Bags and luggage must not obstruct the aisles. A safety valve is normally provided, in case the water separator assembly becomes obstructed by ice. /


obstruction |noun|  1. the act or process of obstructing / The glide-path antenna cannot be placed close to the center line of the runway because it would cause an obstruction. /  2. something which blocks a path or prevents progress / Before start-up, the air intakes and jet pipes must be inspected, to ensure that they are free from any debris or obstruction. / ‘…taxiways and aerodrome obstructions may be hidden by snow, so ask if you are not certain’ [Civil Aviation Authority, General Aviation Safety Sense Leaflet]


obtain |verb| to acquire, to get / Telephone the meteorological office in order to obtain the latest weather forecast. The probes are positioned in the gas stream in order to obtain an accurate temperature reading. /


obvious |adjective| clear and easily seen or understood / It is obvious that high ground will disturb the smooth horizontal flow of air. /


occasion |noun| the time at which an event or happening occurs / In recent months the aircraft suffered two engine failures, on the first occasion the aircraft force-landed safely. The maiden flight of an aircraft is a great occasion. / - on occasions > sometimes


occasional |adjective| happening from time to time - occasional rain > periodic rain - occasional turbulence > turbulence happening from time to time


occluded front |noun| a weather front created when air is forced upward from the Earth’s surface, as when a cold front overtakes and undercuts a warm front / Jet streams are very rare near occluded fronts because of the much smaller temperature gradient across the fronts. /


occlusion |noun| the forcing of air upward from the Earth’s surface, as when a cold front overtakes and undercuts a warm front / If the air ahead of the warm front is less cold than the air behind the cold front, the cold front will undercut the less cold air and form a cold occlusion. /


occupant |noun| a person who has a seat in an aircraft - occupants > the crew plus passengers / In-flight emergency procedures are designed to successfully combat airborne emergencies which threaten the safety of the aircraft and its occupants. /


occupy |verb|  1. to have a position, to be in a place / The passenger is occupying the wrong seat. /  2. to busy oneself / Once an evacuation process is under way the crew will be fully occupied carrying out emergency drills. /


occur |verb| to happen / Heavy rains occur during the monsoon season. Tropical revolving storms generally occur from June to October. An accident occurred on June 12th. /


occurrence |noun| a happening or event / There were a number of occurrences of hijacking in the eighties. The occurrence of the equatorial jet stream is due to a temperature gradient with colder air to the south. /


ocean |noun|  1. the body of salt water which covers the earth (NOTE: This is a chiefly American usage: British English prefers the word sea.)  2. any of the major sea areas of the world (NOTE: The five oceans are: the Atlantic, the Pacific, the Indian, the Arctic and the Antarctic (or Southern).)


oceanic |adjective| referring to the oceans / The trade winds maintain their direction over the oceanic areas, especially the Pacific, more than over land areas. / - an oceanic crossing > a flight across sea or ocean


octa |noun| same as okta


octane rating |noun| the ability of the fuel to resist detonation, i.e. the higher the number, the greater is the fuel’s resistance to detonation


odd |adjective|  1. strange, peculiar / The fact that moist air is lighter than an equivalent volume of dry air seems odd to many people. /  2. - odd tenth > an odd decimal, e.g. 0.1, 0.3, etc. / Frequency allocation of localisers in the VHF band is 108–112 MHz at odd tenths e.g. 108.1 and 109.3, the even decimals being allocated to VOR facilities. /  3. indicating a number a little greater than the approximate number given - it is 60-odd miles to our destination > it is a little more than 60 miles to our destination


odd number |noun| a number which cannot be exactly divided by two, e.g. 1, 3, 5, 7, etc. / A (battery) cell contains an odd number of plates. /


OEM |abbreviation| original equipment manufacturer


offer |noun| something, e.g. a sum of money, that is presented for acceptance or rejection / He made an offer of $85,000 for the aircraft. / - |verb|  1. to show readiness to do something / He offered to pick up the tickets in advance. /  2. to present for acceptance or rejection / The company offered her a job and she accepted it. /  3. to provide / The battery offers a short term power capability. /


official |adjective| referring to an authority, such as the government or a recognized organization - an official weather report > a weather report produced by a meteorological station - |noun| a person employed by a government authority or a corporation / An official of the civil aviation department will be visiting today. /

offload |verb| to remove from the aircraft / As the passenger did not show up at the gate, his baggage had to be identified and  offloaded. /

offset |verb| to shift or displace to the side of a central axis / Crews cannot offset their flight paths in RVSM conditions if parallel route center-lines are less than 30nm apart. /

offshore |adjective| at a distance from the shore


offshore wind |noun| a wind which blows from the coast towards the sea.


ohm |noun| a unit of measurement of electrical resistance. (NOTE: Ohm’s Law states that the current in a circuit is directly proportional to the voltage causing it and inversely proportional to the resistance of the circuit.)


oil |noun| a thick mineral liquid used as a fuel or to make mechanical parts move smoothly


oil pan |noun| US same as sump


okta, octa |noun| a unit of visible sky equal to one eighth of total area visible to the horizon / The amount of cloud cover is given in oktas. / (NOTE: To measure cloud cover, the sky is divided into imaginary sections, each covering one eighth of the total. A cloudless sky is ‘zero oktas’, and a sky which is completely covered with clouds is ‘eight oktas’ or ‘eight eighths’.)


oleo |noun| a telescopic strut in the undercarriage which absorbs impact loads on landing / A safety switch is fitted in such a way to the oleo, that when the oleo is compressed on the ground, the ‘undercarriage up’ selection cannot be operated. / Full form: oleo-pneumatic


OM |abbreviation| outer marker


omit |verb| to leave out, not to include / High charts show only information relevant to high altitude flights and many beacons/aids which are provided for low operations are omitted to keep the chart clear. / (NOTE: omitting – omitted)


omni-bearing indicator |noun| a cockpit instrument that displays VOR information and is used for radio navigation. Abbreviation: OBI


omni-bearing selector |noun| a knob on an omni-bearing indicator which the pilot turns to select a radial from a VOR station. Abbreviation: OBS


omnirange |noun| a very-high-frequency radio navigation network that allows pilots to choose and fly on any bearing relative to a transmitter on the ground

on board |adverb| on the aircraft / We have 358 passengers on board (POB). /

one-in-sixty rule |noun| in navigation, every 1° of track error, and every 60 nautical miles flown, results in the aircraft being 1nm off track

on hold |noun| waiting for authorization or clearance

onshore |adjective| towards the coast


onshore wind |noun| a wind which blows from the sea towards the coast.

on time |adverb| in accordance with the schedule, not late or early, on schedule / The flight arrived on time. /

onward clearance |noun| a clearance to pursue the flight after a way-point or holding action / Expect onward clearance at 09. /

opacity |noun| the state of not allowing light to pass through / Sometimes, it is possible to estimate the depth and opacity of the layer of mist or fog from the ground observations. /


opaque |adjective| not allowing light to penetrate or pass through / Rime ice is an opaque, white, granular ice which forms on leading edges. /


opening |noun|  1. a space which acts as a passage through which something or somebody can go / an inlet valve opening /  2. a formal start of operation / the opening of the new flying school /  3. a vacancy for a job / There’s an opening for a new chief ground instructor. /


open-skies |adjective| referring to a policy of allowing aircraft belonging to any country to fly over an area, without restrictions on surveillance of military installations


operate |verb|  1. to control the working of / The control column operates the ailerons and elevators. The flaps are operated by a switch. /  2. to use or manage / The airline operates a fleet of Boeing aircraft. /  3. to perform or function / Jet transports operate at high altitudes. /  4. to perform a surgical procedure, by cutting into the body / The surgeon operated on the patient. /


operating jack |noun| a device which converts rotary motion into linear or reciprocating motion in order to move heavy control surfaces


operating weight |noun| the total mass of aircraft ready for flight but excluding fuel and payload / The type of undercarriage fitted to an aircraft is governed by the operating weight. /


operation |noun|  1. the process of making something work / The operation of the ignition system in a light aircraft is quite simple. /  2. - long-haul operations > flying over long-distance routes  3. an effect - to come into operation > to come into effect / The new procedures come into operation on 1st January. /  4. a surgical procedure / The doctor performed an operation. /  5. a procedure such as addition or subtraction


operational |adjective|  1. working or functioning / Air traffic control facilities were not operational at the time of the accident. / - the operational life of the aircraft > the expected working life of an aircraft  2. ready for use, referring to an aircraft in a suitable condition to fly - an operational aircraft > an aircraft that can be used for its assigned purpose


operational air traffic |noun| flights operating in accordance with military air traffic service procedures. Abbreviation: OAT


operations department |noun| the part of an airline or airport organization which deals with flight operations


operative |adjective| functioning or working / The system is now operative after the recent maintenance. /


operator |noun| a person who operates or uses equipment / A ring graticule around the edge of the cathode ray tube enables the operator to read the bearing directly. /


oppose |verb|  1. to work against / In level flight, the force of lift opposes the force of gravity. /  2. to reject, be in conflict with or try to prevent / The local people oppose the building of the new runway. /  3. - as opposed to > in contrast with


opposite |adjective|  1. situated or placed directly across from something, facing - opposite sides of a building > the back and front of a building  2. completely different, the reverse / For every action there is an equal and opposite reaction. / - |noun| something completely different, the reverse / The opposite of a katabatic wind is an anabatic wind. The opposite of starboard is port. / - going in opposite directions >  1. moving away from each other  2. moving towards each other


opposition |noun| - in opposition > against / Drag acts in opposition to thrust. The electromotive force that is produced by all motors is in opposition to supply voltage and is directly proportional to motor rpm (revolutions per minute). /

OPS |abbreviation| Operations, flight ops

optimum |adjective| referring to the point at which the condition or amount of something is the best / The optimum altitude for jet aircraft is higher than that for piston engine aircraft. / - |noun| the point at which the condition or amount of something is the best / Generally speaking, engine output is at its optimum at cruising speed. /


option |noun| a choice or alternative / On a bad approach, the pilot of a powered aircraft always has the option of going around. He was given the option of buying two aircraft instead of one. /

orbit |noun| the path of a planet, or of a satellite as it moves around another celestial body / A year is the time taken for the Earth to complete one orbit round the sun. / - to put into orbit, to send into orbit > to launch something into space so that it revolves around a celestial body such as the sun - |verb|  1. to revolve around / The Earth orbits the sun. / (NOTE: orbiting – orbited)  2. to perform a 360° circuit usually in order to delay / For safety reasons, the ATC will tell the pilot whether to orbit left or right. /

order |noun|  1. an instruction given as a command by somebody in authority / The captain gave the order to evacuate the aircraft. /  2. the sequence of occurrence / The firing order of sparking plugs in a piston engine is 1, 3, 4, 2. / - alphabetical order > arrangement in which words beginning with letter A come first, followed by those beginning with letter B, then C, etc. - numerical order > arrangement in which the lowest numbers (1, 2, 3, etc.) come first and higher numbers (25, 26, 27, etc.) come later  3. a condition or state / Although the aircraft is old, it is in good working order. / - out of order > not working / The telephone is out or order. /  4. - in the order of > approximately / VOR (very high frequency omni-directional radio range) beacons of 200 watts have a range in the order of 200nm (nautical miles). / - in order to > so as to / Indicated airspeed must be corrected in order to obtain true airspeed. / - |verb|  1. to give a command / Before impact, the captain will order the crew to secure themselves at their assigned emergency stations. /  2. to put in a sequence / Order the items in importance from 1 to 10. /


organization |noun| 1. an association of people working together for the same cause / The World Meteorological Organization, The International Civil Aviation Organization /  2. the act of putting things into a structured and systematic form / The organization of training materials for the new self-access learning center is under way. /  3. planning / Captain Scott is responsible for the organization of examinations. /


organize |verb|  1. to arrange into a system / Organize your notes so that you can find things easily. /  2. to plan / The trip was well organized and everybody enjoyed themselves. /


orientate |verb| to locate in relation to the compass / The first step in map reading is to orientate the chart by relating the direction of land features to their representation on the chart. /


orientation |noun| a position in relation to the compass / The horizontal situation indicator (HSI) presents a selectable, dynamic color display of flight progress and plan view orientation. /


orifice |noun| an opening, mouth or vent / The liquid expands and builds up a pressure differential across an orifice which leads to the expansion chamber. /


origin |noun|  1. a source, the place where something starts / An air mass takes on the characteristics of its place of origin. /  2. the base from which a map projection is drawn / The value of convergence used is correct at the parallel of origin. /


original |adjective| before all others, the first / The atmosphere is said to be stable if, when a parcel of air is displaced vertically, it tends to return to its original level. /


originate |verb| to be created or to come into being / Tropical revolving storms originate within 5–15° of the equator. Aircraft fires after an emergency landing, often originate in the wing area. /


orographic |adjective| referring to mountains - orographic uplift > the lifting of air masses in contact with mountain regions


orographic cloud |noun| a cloud formed by air being forced upward over mountainous areas


orthomorphic |adjective| of the correct shape / An orthomorphic chart is one which has meridians and parallels which intersect at right angles and, at any point on the chart, the scale must be the same in all directions. /


orthomorphism |noun| a shape representation on a map / Orthomorphism means that bearings may be measured correctly at any point on a chart. /


oscillate |verb|  1. to move regularly between extremes  2. to increase or decrease regularly so as to produce oscillations / Instability protection is incorporated to guard against oscillating outputs from the alternators. /


oscillation |noun|  1. a regular movement between extremes / Ridge waves can be thought of as oscillations about the stable state of the undisturbed air flow with the range of hills providing the disturbance. /  2. a regular increase and decrease of electrical current / The supply is subject to oscillation. /


oscillator |noun| an electronic circuit that produces a pulse or a signal at a particular frequency / The local oscillator replicates the radio frequency of the frequency generator at the transmitter. /


out |adverb| - out of > away from, no longer in - out of phase > not following, in harmony or aligned with something else


outboard |adverb| in a direction away from the center of an aircraft - |adjective| situated away from the main body of an aircraft and towards the wing tips


outbound |adjective|,|adverb| towards a destination away from a VOR / The aircraft flies outbound from the beacon along the airway and inbound to the facility at the other end of the leg. / - outbound traffic > aircraft flying away from an airfield

outbound destination sign |noun| an airport sign which indicates the direction to common taxi routes

outbound track |noun| a flight away from a NAVAID fix

outbreak |noun| a sudden start / Showers are local outbreaks of precipitation from detached cumulus or cumulonimbus. Hand operated fire extinguishers are provided to combat any outbreaks of fire in the flight crew compartment and passengers cabins. /


outer |adjective|  1. external / Pneumatic deicer boots are made from vulcanized rubber fabric with an outer covering of neoprene. /  2. positioned away from the center / Winds near anticyclones are normally light near the center, but tend to be stronger towards the outer edges. /  3. - outer wing > the part of the wing nearest the tip


outer marker |noun| an ILS marker beacon, usually on center line of approach at about 4.5nm from the runway threshold

outer taxiway |noun| a continuous taxiway between an apron and inner taxiways and runways allowing aircraft to circumnavigate (taxi around) the terminals for easier access

outer windshield panel |noun| an external layer of a windshield; aircraft windshields comprise up to five separate layers, incorporate gold heating filaments and may weigh up to 80kg

outflow |noun| flow in an outward direction / The outflow valve is controlled by the cabin pressure controller. /


outgoing |adjective| going out / There is a fall of temperature until about one hour after dawn when incoming solar radiation balances outgoing terrestrial radiation. / Opposite: incoming


outlet |noun| a passage for exit or escape / The air leaves the compressor outlet and passes through a matrix assembly of the secondary heat exchanger. When the controlling super-charger outlet pressure is reached, the capsule is compressed sufficiently to open its bleed valve. /


outline |noun|  1. a line around the shape of something / Warning labels have a solid red outline. /  2. a shape / At low level, features are most easily recognized from their outline in elevation. Cumulus cloud has detached domes or towers which are generally dense and have sharp outlines. / - |verb| to explain simply and briefly / The changes in conditions outlined in the next paragraph. /

outlying stand / remote stand |noun| a parking position which is not directly connected to terminal

out-of-balance turn |noun| a turn in which the aircraft ‘skids’ upwards and outwards from the turn or ‘slips’ inwards and downward / During an out-of-balance turn, the ball in the slip indicator will be deflected to the left or right. /


out of trim |adjective| referring to a situation in which the aircraft is not in static balance in pitch, so that if the pilot releases the yoke or control stick, the aircraft will start to climb or descend


output |noun| the product of a process / Air density will affect the output of the engine. The function of the supercharger is to increase the power output. The power output of an engine depends on the weight of mixture which can be burnt in the cylinders in a given time. /


outrigger |noun| a projection attached to an aircraft to stabilize it or to support something


outward |adjective| moving away from the center or starting point / The piston draws fluid into the cylinders on the outward stroke and expels fluid into the system on the inward stroke. /


outwards |adverb| away from the center or starting point, towards the outside / The door opens outwards. / (NOTE: The US English is outward.)

OVC |abbreviation| an overcast

overall |adjective| including everything / The total aerodynamic losses result in an overall turbine efficiency of 92%. Although the student failed in one of the five exams, her overall result was a pass. / - |adverb| generally / Overall, the test flight was a success. / - |noun| a one-piece item of protective clothing / The engineer was wearing an overall to prevent his clothes from getting dirty. /


overalls |plural noun| protective trousers with a bib and straps over the shoulders / Wear overalls to protect your clothes. /

overcast |noun| a complete cloud cover, 0.9+ (i.e. over 90%) cloud cover. Abbreviation: OVC

overcome |verb| to beat, to conquer, to win against / The effects of anoxia at high altitudes can be overcome by breathing through a mask. Drag must be overcome with thrust in order for an aircraft to increase speed. /


overflight |noun| the flight of an aircraft over an area


overfly |verb| to fly over an area (NOTE: overflew – overflown)


overhang |noun|  1. the distance from the last outer strut to the end of a monoplane’s wing  2. a distance equivalent to half of the difference in the spans of the two wings of a biplane


overhaul |verb| to take apart and examine carefully in order to repair and clean, etc. / To overhaul the system will take a couple of days. / - |noun| the act of taking apart in order to repair and clean / Other than the oil pump and the generator rotor, there are no other moving parts in the system to wear or which require periodic overhaul. /


overhead |adjective|  1. vertically above the point where a course is measured or timed / The aircraft started from overhead A at 1000 hours on a heading of 230°T. /  2. above the level of people’s heads / Overhead baggage lockers must be secured immediately prior to take-off. / - |adverb| above one’s head / She noticed a plane flying overhead. /

overhead panel |noun| an instrument panel above the pilots’ heads in the cockpit which contains most of the system control panels on aircraft with a two-man crew

overheat |verb| to get too hot / An acceleration/deceleration control is fitted to prevent the turbine assembly from overheating during acceleration, and to prevent flame-out during deceleration. /


overlap |noun| part of one thing covering something else - |verb| to have an area or range in common with something else, or to cover part of something else / The maps overlap each other at the edges by three centimeters. / (NOTE: overlapping – overlapped)


overload |noun| an excessive amount of work or electricity / Resettable circuit protective devices should be designed so that when an overload or circuit fault exists, they will open the circuit. / - |verb|  1. to load a device or system, such as an electrical circuit, with too much work; to demand more than a system is capable of / Operating pressure is maintained in that part of the system which leads to the selector valves, and some method is used to prevent overloading the pumps. /  2. to load too heavily / The aircraft failed to gain height after take-off because it was overloaded. /


overload operations |noun| operation of aircraft in unusual situations when take-off weight exceeds the permitted maximum

over-reliance |noun| depending on somebody or something too much, which creates dangers when that person or thing is unavailable or wrong

override |verb| to take over control of the operation of an automatic device or system / A circuit-protective device must not be of a type which can be overridden manually. / (NOTE: overriding – overrode – overridden)


overrun |noun| a cleared level area at the end of a runway, available in case a plane does not stop quickly enough


overshoot |verb| to fly past a target / The pilot tried to land but the aircraft overshot the runway. / (NOTE: overshooting – overshot)

overshoot wind-shear |noun| a wind-shear characterized by an increase in aircraft airspeed

oversight |noun|  1. as an uncountable noun, oversight means supervision, from the verb ‘to oversee’ / The Federal Aviation Authority has oversight of air traffic control and safety regulation within US airspace . /  2. as a countable noun, an oversight is something that has been forgotten or missed / Not removing the pitot covers was an oversight by maintenance. /

over-speed |verb| to go too fast / A fault in the constant speed drive unit causes the generator to over-speed. / - |noun| a speed that is too fast / Over-speed is usually a fault in the constant speed drive unit which causes the generator to over-speed. /


over-speeding |noun| the act of going too fast / Over-speeding of the engine is prevented by a governor in the fuel system. /


overstress |verb| to subject to too much force /It takes less g force to overstress a heavy aircraft than a light one./


owing to |preposition| because of / Integral tanks are now favored for aircraft owing to the very high utilization of space and saving of weight. Owing to the aerodrome being unserviceable, the landing was made at another aerodrome some distance away. /


oxidation |noun| the combination of a substance with oxygen, with loss of electrons / When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin forms immediately that protects the metal from further oxidation. /


oxide |noun| a compound of an element with oxygen / When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin forms immediately that protects the metal from further oxidation. /


oxidize |verb| to form an oxide by the reaction of oxygen with another chemical substance / Over a period of time, the metal is oxidized by contact with air. /


oxygen |noun| a colorless, odorless gas, which is essential to human life, constituting 21% by volume of the Earth’s atmosphere / Our bodies can get oxygen through the lungs. At very high altitudes the flying pilot must be on oxygen at all times, unless an aircraft dispensation has been obtained. / (NOTE: The atomic number of oxygen is 8.)

ozone |noun| a poisonous form of oxygen found naturally in the atmosphere which is toxic to humans at concentrations above 0.1 parts per million / The maximum concentration of ozone is between 20 and 25 km above the Earth’s surface. / Symbol: O3

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