Although passengers on commercial airline flights share concerns about airfares, on-time performance, cramped seats, and lost baggage, there is far more behind the process of flying between two cities. Indeed, it is more of an orchestration between airlines, airports, governmental agencies, and numerous other companies. The following sequence of events illustrates this.
1. At the Airport:
Preparations for a schedule or chartered airline flight, whether it be a one-hour hop or an intercontinental journey of 15 or more hours, begins long before the passenger departs for the airport and the aircraft itself touches down from its prior segment.
Passengers themselves are progressively checked in and their baggage is tagged, sorted, and routed. Cargo is weighed and manifested. Aircraft are cleaned, catered, serviced, and maintained.
The cockpit and cabin crew arrive at the airport, conducting briefings, but the former reviews any special load, the flight plan, and the weather, and calculates the final fuel, which includes the minimum required for the journey, along with that for reserves, holds, and diversions.
Tantamount to this process is completion of either a manual or computerized load sheet, which lists and builds upon the aircraft’s various loads and ensures that none exceed their maxima.
An Airbus A-330-200, for instance, with 15 crew members, would have a 124,915-kilo dry operating weight (DOW), to which its calculated take off fuel (TOF) of, say, 77,600 kilos, would be added, giving it a 202,515-kilo operating weight. An 18,750-kilo passenger load, comprised of 85 males, 161 females, one child, and one infant, would be added to its 8,085-kilo dead load, itself consisting of 4,320 kilos of baggage, 3,630 kilos of cargo, and 135 kilos of mail. Combined with the previous total passenger weight, it would result in a 26,915-kilo dry operating weight, which, added to the 124,915-kilo dry operating weight, would produce a 151,750-kilo zero-fuel weight.
Now added to its 77,600-kilo take off fuel, this A-330 would have a 229,350-kilo take off weight, which is just shy of its 230,000-kilo maximum. After in-flight burn of its 68,200-kilo trip fuel, it would have a 161,150-kilo landing weight, which itself is well below its 182,000-kilo maximum.
Aside from hinging upon the many previously discussed functions, the final fuel uplift additionally depends upon captain discretion. An aircraft with a 56,200-kilo final block fuel, for example, would result in a 55,800-kilo take off fuel, after the estimated 400 kilos of taxi fuel was burned, and the 44,900-kilos required for its flight plan would give a New York-Vienna A-330-200 flight a seven-hour, 12-minute enroute time, but an eight-hour, 28-minute endurance (to dry tanks).
2. At the Gate:
The weight and balance function, from which these calculations derive, implies both the load sheet’s weight build-ups and calculations and the distribution of its traffic load, and ensures that the aircraft is loaded within its safe center-of-gravity (CG) envelope, while in-flight balance is achieved by the setting of its stabilizer trim. Although this is automatically determined in the cockpit, it can be manually calculated, as can occur with Boeing 767 aircraft.
All these calculations additionally determine take off speeds and flap settings.
Although the aircraft’s position was recorded and stored in its inertial navigation system (INS) when it arrived from its last sector, along with the compass direction of true north without magnetic variation and the earth’s shape and movement, it is realigned and re-entered, adhering to the terminal building’s latitude and longitude coordinates, expressed in degrees north, south, east and west, and minutes. JFK’s position, for instance, is 40 degree, 38.9 minutes north latitude and 076 degrees, 46.9 minutes west longitude.
Two important clearances precede aircraft movement: the first, from clearance delivery, enables it to accept and pursue its flight plan, and the second, from the tower-located ground control, gives it permission to taxi to the active runway’s holding point. Push-back clearance, actually the first, is granted by the terminal’s own tower, which monitors arriving and departing movements from and to the taxiway to its ramp, over which it has jurisdiction.
Headphone-connected to the aircraft’s external port, maintenance or ramp personnel monitor engine start either during push-back, which is achieved by a towbar-connected tug, or on the ramp. Some airports, such as Atlanta-Hartsfield International, permit autonomous power reverse thrust movements of narrow body aircraft.
During taxi itself, which is not unlike an automobile’s ground movement and attained by means of a throttle advance, movement of the nose wheel steering tiller located on the captain’s lower left side, and toe brake applications, the taxi and pre-take off checklists are completed in the cockpit and exit and oxygen mask demonstrations are given by flight attendants or prerecorded films in the cabin.
Because the main wheels are located a significant distance from the nose wheel, ground turns are almost made at 90-degree angles.
A two-bell chime indicates imminent take off.
4. Take Off:
Issued one or more clearances, such as “hold short of,” “move into position and hold,” and/or “cleared for takeoff,” from the tower, the aircraft positions its nose wheel on the runway’s center line. Lighting indicates length: amber marks the last 3,000 feet, red and white the last 2,000, and all-red the last 1,000.
Take off throttle settings vary according to aircraft gross weight, runway length and surface conditions, the need to clear obstacles, and the desire to prolong engine life.
Jet engine thrust is created by the reaction principle, as expressed by Sir Isaac Newton’s third law of motion, which states that “for every action there is an equal and opposite reaction.” Despite what may initially seem complex, the pure-jet engine (without external propellers) entails a four-stroke process of air intake; combustion, during which it is mixed with fuel and heated and then it expands; compression, at which time it turns both the turbine and the compressor by means of a connecting shaft; and exhaust.
As the air exits, it reacts by pushing the aircraft forward and generating thrust.
A turbofan consists of the traditional powerplant components and a forward, shrouded fan, whose rotating blades send slower, cooler, and quieter air around the hot core before it exits through the exhaust cone. This is expressed by the term “bypass ratio,” which is the ratio of thrust created by the bypassing air as opposed to that generated by the hot section. A typical high bypass ratio turbofan could have a bypass ratio of about five-to-one and can generate up to 100,000 pounds of thrust on aircraft such as the Boeing 777-300ER.
Although cockpit instrumentation, such as N1 fan speed and fuel flow, register powerplant parameters, one of the most important is its engine pressure ratio (EPR), which is the ratio between the turbine discharge and compressor inlet pressures.
There are three important, pre-calculated take off speeds, which vary according to aircraft, engine capacity, gross weight, runway length and surface conditions, atmospheric conditions, and power settings.
The first, V1 (for velocity), is the go or no-go speed. If an anomaly or emergency occurs at this point, there is still sufficient runway length remaining in which to safely stop.
The second, VR, is the aircraft’s rotation speed, at which point the stabilizers are deflected so that the airplane can “rotate” on its main undercarriage, increasing its angle-of-attack (AOA) to the onrushing air, and achieve lift. Insufficient runway remains for an aborted take off at this point. If one is attempted without the presence of an arrestor bed, overruns and potential damage and injury are to be expected.
The third speed, V2, is the safe climb-out velocity, which is usually calculated as V2 + 10 knots.
All, of course, vary according to aircraft type, version, and engine, but V-rotation speeds include 137 knots for a McDonnell-Douglas DC-9-30 with Pratt and Whitney JT8D-15 engines, 147 knots for a Boeing 737-200 with JT8D-15As, 134 knots for a Fokker F.28-4000 with Rolls Royce RB.183-555-15A Spey engines, 184 knots for a McDonnell-Douglas DC-10-30 with General Electric CF6-50C2s, 165 knots for a Lockheed L-1011-200 with Rolls Royce RB.211-524B turbofans, 177 knots for a Boeing 747-300 with Pratt and Whitney JT9D-7R4G2 turbofans, and 153 knots for an Airbus A-300-600R with General Electric CF6-80C6A5 turbofans.
None of this, needless to say, would be possible without the lift generated by the wing. Created by the pressure differential between its upper and lower surface, the former is reduced by the air passing over it and the downwash it produces as it adheres to the boundary layer over its trailing edge.
It can be illustrated by the simple physics principle, which states that “as speed increases, pressure decreases.” Since objects always take the path of least resistance, the wing moves upward, generating lift. This can also be augmented by several other factors, including wing planform (shape), aspect ratio (the ratio of its length to its width), sweepback, area, speed, temperature, and air density.
To further improve their capabilities, commercial jetliners increase lift at slow speeds, yet reduce drag at higher ones, by employing both area- and camber-increasing leading edge slats/flaps and trailing edge flaps, usually of the Fowler type, which vary from single- to double- to triple-slotted ones. Full extension of both, a configuration only used during landing, increases the wing’s area by 20 percent and its lift capability by 80 percent.
With the exception of the Airbus A-300, few modern jets are able to take off without some degree of trailing edge flap extension.
Immediately after take off, an aircraft will be instructed to contact departure control, which is usually located at the base of the control tower in a windowless facility and can provide altitude clearances and traffic-separating radar vectors. A common instruction would be, “Trans-Atlantic one-six-zero heavy, climb and maintain one zero thousand.”
Aircraft follow prescribed, airport-departing courses known as “standard instrument departures” or “SIDs.” The Ventura Seven departure from Runway 24R at Los Angeles International Airport, for example, entails a “climb on heading 251 degrees for radar vectors to (the) VTU VOR/DME, cross SMO (Santa Monica) R-154 (154-degree radial) at or below 3,000, (thence) continue (on assigned route). All aircraft expect further clearance to filed altitude five minutes after departure.”
Similarly, the Compton 2G standard instrument departure from Runway 27L at London’s Heathrow International Airport, employing a 123.9-MHz (megahertz) frequency, requires an initial, runway heading climb from the London VOR, then at seven miles DME (distance measuring equipment) a right turn to track 273 degrees to the Woodley NDB (non-directional beacon). Finally, the aircraft must maintain a 285-degree heading to the Compton VOR, but not climb above 6,000 feet unless given prior clearance to do so.
After a positive climb rate has been established only minutes after disengagement from the ground, the airliner’s undercarriage is retracted, at about 1,000 feet, usually requiring a decreased nose pitch.
Nose wheel rotations are cancelled by a wheel well snubber, while those of the main wheels cease with a brake application.
All aircraft operating near airports with Terminal Control Areas (TCAs) must have transponders and encoding altimeters, and the air traffic control provided four-digit code identifies it on radar.
Based upon gross weight and speed, the leading and trailing edge high-lift devices are often fully or progressively retracted, leaving the wing “clean.”
Now established on its flight plan, the aircraft will be handed off to an air route traffic control center (ATCC) with radio transmissions such as, “Boston Center, this is Trans-Atlantic one-six-zero, with you at flight level three-five-zero.”
In the cabin, in-flight service is likely to have begun.
The altimeter, whether in the form of the older, traditional “steam gauge” or the new, cathode ray tube (CRT) display, indicates the aircraft’s altitude and height, but they are not necessarily the same, and both can vary widely between take off and touchdown.
As an airplane climbs, static pressure, fed to an altimeter case, reduces and the capsule within the instrument expands, transmitting this change, via a mechanical linkage, to the cockpit instrument, thus measuring and displaying height, which, perhaps surprisingly, can have little relation to the aircraft’s actual height.
If, for example, it flies at 5,000 feet, it may be at exactly that altitude above mean sea level (MSL), but if it is passing over a 2,000-foot mountain, it is really only 3,000 above ground level (AGL), which, needless to say, may rapidly change as it continues to cruise, especially at high speed.
Topographical variations, from a small lake to Mount Everest, are countless, and the equality of height and altitude only occurs when it is above sea level.
In order to improve accuracy, a radio altimeter, which bounces radio waves off of and then measures the exact height above ground elevations, is used during approach, when the aircraft is usually at or below 2,500 feet.
While the pressure altimeter is set to equal the pressure of the intended airport, it is given the standard pressure setting in cruise, which is 29.92 inches of mercury (in. Hg) in North America and 1013.2 millibars (mb) elsewhere.
Altitudes above 28,000 feet are considered flight levels (FLs). 36,000 feet is therefore flight level three-six-zero, which omits the last two digits.
Speeds also vary, but are measured in knots, equivalent to a nautical mile. Ground speed (GS) is a measurement of an airplane’s speed relative to the ground, while its true air speed (TAS) is its speed relative to the air through which it passes. Wind speeds and direction cause the variations.
Navigation, adhering to an aircraft’s air traffic control, instrumental flight rules (IFR) flight plan, is achieved by a number of methods. The first of these is the VOR.
Transmitting a very high frequency (and thus its “VOR” designation) band from the 108.0 to 117.9 MHz frequency, it provides line guidance to and from the more than 700 stations in the US alone, creating reporting or waypoints, which, when linked, produce the “victor” airways below 18,000 feet and the “jet” ones above, that airliners ply.
VOR signals can be received up to 230 nautical miles away, potentially creating a 460-mile airway, and an airplane’s position to or from is considered, respectively, an in- and outbound radial, as it first flies toward and then away from it.
Displays so indicating consist of a radio magnetic indicator (RMI) needle, which points toward the ground-based beacon, and a vertical orange bar on the aircraft’s main compass system.
Standardly co-located with a VOR beacon is distance measuring equipment (DME), which transmits in the ultra-high frequency (UHF) band from 962 to 1213 MHz and provides a digital readout, via the aircraft’s transponder, of its slant-range distance to the beacon itself.
The latest and most accurate navigation method is that created by the 32 earth-orbiting global positioning satellites (GPS), which determine it by means of the time difference between signal sending and receiving within a triangulation process, involving three, atomic clock-equipped satellites themselves, yielding a range readout.
Use of four such satellites, which eliminates the ionosphere- and troposphere-caused delays, results in latitude, longitude, altitude, and time determinations.
Because a flight plan’s coordinates may only be spaced 25 miles apart, a 5,000-mile sector could consist of some 200 of them, or vastly more than the VOR/DME waypoints. Nevertheless, they tick off as the aircraft proceeds, with the estimated time enroute (ETE) and fuel burn calculated between them.
Atlantic crossings, by means of latitude and longitude coordinates, are under the jurisdiction of the Atlantic Control Area, which is subdivided into the western expanse from Greenland to the Caribbean and controlled by Gander and New York, and the eastern from 300 degrees and is controlled by the combined Shannon and Prestwick air traffic area designated Shanwick.
To accommodate the unprecedented number and frequencies of trans-Atlantic crossings, a half-dozen east- and westbound, parallel North Atlantic tracks, based upon the best available routes and most favorable winds, are published twice daily.
Lettered, the eastbound ones include track alpha, bravo, Charlie, delta, echo, and foxtrot. Valid for crossings between the Canadian and Ireland/United Kingdom coasts, they consist of ten-minute same and 60 nautical mile different track longitudinal separations and 2,000-foot same and identical altitude different vertical separations.
Outboard, high-speed aileron locks ensure minimum cruise bank angles. Although altitudes are flight plan assigned, gross weight, engine thrust, and wing capability may initially require step-climbs–that is, progressively higher altitudes facilitated by fuel-burn and weight reductions, although crews can request different flight levels to avoid or minimize weather- and turbulence-caused passenger discomfort. Even different tracks can be requested, if available.
Fuel consumption and throttle settings vary according to the aircraft, its gross weight, altitude, and flight mode. During take off, a full throttle to the take off/go-around (TOGA) detente may result in a 32-ton-per-hour fuel burn on a Boeing 747-400, although this setting may only be maintained for a scant few minutes until it is airborne, while it would be reduced to a third, of between ten and 12 tons, in cruise.
Despite its advancement, the Aerospatiale-British Aerospace Concorde, because of the rarefied air in which it had supersonically cruised and the unfeasibility of installing large-diameter turbofans in its wing root configuration, maintained full throttle settings of its Rolls Royce, afterburner-equipped Olympus engines throughout its entire flight. Slower, cooler bypassing air would have been ineffective in thrust production in such an atmosphere. Yet, in order to avoid nose and airframe over-temperature limits, it was given a block altitude in which it could slowly climb as fuel burn-weight reductions would otherwise have eclipsed its never-exceed speed.
Integral and indispensable to any modern jetliner is the flight management system (FMS). Located on the cockpit’s center console between the captain and the first officer, it offers integrated inertial navigation, performance, and fuel management functions, enabling countless pages of information to be either inserted or requested by means of a numbered and lettered keyboard, including airport SIDs and STARs, and the aircraft’s origin-to-destination flight plan.
Through the system, the autothrottle maintains the proper engine pressure ratio in its VNAV (vertical navigation) mode during climb, cruise, and descent. Much more than that, however, it receives and processes a barrage of sometimes beyond human brain capability information, such as N1 fan speed, fuel flow, and ground speed, always providing the optimum performance in accordance with the selected parameters, like best time, minimum fuel, maximum range, and lowest cost.
6. Descent and Landing:
Receiving its initial descent clearance from the air route traffic control center, whose jurisdiction it is presently under, the aircraft begins a one- to two-degree, 1,000-fpm altitude loss, now encased in increasingly louder slipstream, while flight attendants secure the cabin, closing all galley compartments and drawers, ensuring that seats are in their upright positions, that seatbelts are fastened, that carry-on luggage does not obstruct any aisles, that the overhead storage compartments are latched, and that class curtain dividers are open, and perhaps returning coats and other garments to first and business class passengers.
Cabin pressurization is set to equal that of the arrival airport. The automatic terminal information service (ATIS), a pre- and progressively-recorded message concerning cloud coverage, ceiling, visibility, type and extent of precipitation (if any), temperature, dew point, wind velocity and direction, barometric pressure altimeter setting, the active runway(s), the type of instrument approach, and special information, like field conditions, taxiway closures, inoperative facilities, and wind shear, is accessed and lettered as updates necessitate, resulting in “information alpha,” “information bravo,” “information Charlie.”
Clearance, below 14,000 feet, has already been handed off to the respective arrivals control. Throttle settings are most likely at idle.
As had occurred after departure with the standard instrument departure, or SID, approaches have their counterpart–standard arrival routes or STARs, although air traffic control vectoring to increase spacing because of poor weather and/or peak traffic conditions, as well as placing aircraft in holds over VORs, during which they pursue four nautical mile racetrack patterns, are common.
The Dekal Four Arrival at Fort Lauderdale/Hollywood International Airport, for example, lists three transitions from Freeport, Nassau, and Ursus. The middle one, from Nassau, Bahamas, entails following the 315 radial on a 300-degree heading and at a 6,000-foot altitude from the ZQA VOR/DME, before intercepting the ZBV 100 radial to the ZBV VORTAC (a navigational aid consisting of a co-located VHF omnidirectional range–VOR–beacon and a tactical air navigation system–TACAN–beacon), continuing on the 100 radial from Carey on a 280-degree heading, still at 6,000 feet, and finally intercepting the 300 radial on a 300-degree heading and maintaining 4,000 feet to Dekal. “Thence, from over Dekal, expect radar vectors to final approach course.”
Approach types vary from visual, VOR/DME, ILS, and coupled visual-ILS, among others.
The ILS, the instrument landing system, provides a precision approach from ten to up to 50 nautical miles, at a three-degree glideslope, to runways equipped with both a localizer, which is a radio signal offering center line guidance, and the glideslope itself, which facilitates profile guidance to the touchdown point.
Three ILS marker beacons, transmitting on a 75-MHz frequency, indicate the remaining distance to this point: the outer marker (OM), located at about five nautical miles from touchdown, the middle marker (MM) at between one and 0.5 miles, and the inner marker (IM) at the threshold.
Threshold speeds vary with aircraft, such as 137 knots for a Boeing 727-200 with Pratt and Whitney JT8D-17 engines, 142 knots for a McDonnell-Douglas DC-8-71 with CFM-56-2-C5 turbofans, 146 knots for a Lockheed L-1011-500 with Rolls Royce RB.211-524Bs, and 141 knots for a Boeing 747-100 with General Electric CF6-45A2s.
Having already been handed off to the airport’s tower, maintaining an approximate 1.2 engine pressure ratio reading, and capturing the instrument landing system, the aircraft extends its area- and camber-increasing leading and trailing edge devices, assuming an ever-shallower sink rate, from perhaps 100- to 50-fpm. Spoilers are armed to extend upon main wheel compression and brakes can be set for the optimum stopping distance according to runway length, touchdown speed, and surface conditions, such as water or ice.
Passing over the outer marker, it is issued its “cleared to land” instructions.
Maintaining a 1.14 engine pressure ratio and having its altitude, heading, speed, descent rate, power settings, and time to touchdown closely crew monitored, it passes over the threshold, its height either called out by the non-flying pilot or automatically announced.
“50 feet… 40… 30.”
Pitching its nose up and profiling it for main wheel contact, the aircraft sinks the last few feet.
“20 feet… 10… retard (the autothrottles close).”
Upper wing surface spoiler panels deflect by as much as 60 degrees, impeding the air flow’s lift generating capability and transferring the airplane’s weight to its wheels. Reverse thrust, usually by means of clamshell doors, is activated when the exhaust is laterally and then forwardly vectored, reducing the deceleration run and minimizing brake usage, which generates considerable heat. steering is attained with the rudder, via the rudder pedals.
A forward yoke pressure enables the nose wheel to make runway contact, with steering itself transferred to its tiller at about ten knots.
Contacting ground, it receives its taxi instructions to its gate or parking stand, where passengers disembark, their baggage is sent to the arrival carrousel, and cargo is transferred to the warehouse for sorting.
Poised to operate its next sector, which could occur in 30 minutes or in several hours, the aircraft will be subjected to the same process all over again.