1 Operational Details of the Air Transportation System · 4 Handbook of Human Factors in Air Transportation Systems GROUND OPERATIONS Controlled airports, where air transportation

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1 Operational Details of theAir Transportation System

Steven J. Landry

INTRODUCTION

Air transportation systems worldwide consist of several common features—aircraft are parked at airports, taxi from the parking area to a runway, use the runway to take off, then fly to some desti-nation airport where the aircraft lands at a runway, taxis back to a parking area, and parks. There are substantial variations in the operational details of how this works under different conditions, including the experience of a passenger on a commercial flight, which is how most people experi-ence the system, passengers on modern private aircraft, and passengers or pilots in older general aviation aircraft. Although much of the current chapter is focused on the common commercial flight experience, notations are provided as to what differences one can expect across different types of operations.

Much more of the technical details of the air traffic control system is contained in Chapter 3. This chapter gives an overview of what is involved operationally in moving an aircraft from one place to another within an air transportation system, to bring readers up to speed on the various elements of the system, and how they go together in an operational sense. This chapter, of course, is not a replacement for the actual air traffic control regulations, which are more complete, up-to-date, and authoritative. See especially Federal Aviation Administration (2016).

One can generally separate an aircraft’s ground operations, shown schematically in Figure 1.1, from its airborne operations, shown schematically in Figure 1.2, which appears after the next sec-tion. These figures are labeled; those labels correspond to the references in the following subsections.

CONTENTS

Introduction ........................................................................................................................................3Ground Operations .............................................................................................................................4

The “Gate” (A, B) .........................................................................................................................5Pushback (C) .................................................................................................................................5Taxi Out (D) ..................................................................................................................................6Taking the Runway (E, F) .............................................................................................................6Takeoff (G) ....................................................................................................................................6Arrival Ground Operations (S–V) .................................................................................................7

Airborne Operations ...........................................................................................................................7Climbout (H–K) ............................................................................................................................8Cruise (L) ......................................................................................................................................8Descent (M, N, O) .........................................................................................................................9Arrival (P, Q) .................................................................................................................................9Landing and Go-Around (R) ....................................................................................................... 10

Conclusion ....................................................................................................................................... 10References ........................................................................................................................................ 10

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4 Handbook of Human Factors in Air Transportation Systems

GROUND OPERATIONS

Controlled airports, where air transportation authorities manage the operations of the airport, typically have one or more ground controllers who perform the functions that will be discussed in the next few sections. At very busy airports, there will be numerous ground controllers, each with different responsibilities; at less busy airports there may be one or even none. These ground control-lers typically sit in an air traffic control tower, where they can watch the ground operations visually. Sometimes, there is more than one tower, with separate towers located where ground controllers can see parking areas that would otherwise be obscured from the main tower.

At very busy airports, where one commercial airline has a majority presence, some parking areas may be controlled by a specialized ramp controller who may be employed by the airline. That controller’s responsibility is to manage the gate and initial taxi process in that particular ramp area.

Moreover at controlled airports there are tower controllers, who manage operations on the run-way, including approving landings and takeoffs. These controllers are usually separate from the ground controllers and focus their entire attention on the runway environment. These controllers are in the tall control tower seen at all busy airports, and operate almost exclusively visually, eschewing most computer-based decision support systems that would distract them from seeing operations at the runway.

At airports that are not controlled, there may be no controllers at all. In such cases, pilots are responsible for decisions regarding taxi and takeoff.

D

C

E

FGS

T

A,BV

U

GateGate

FIGURE 1.1 Ground operations overview.

H

N

O

M

P

L

KJ

J

IRQ

K

FIGURE 1.2 Airborne operations overview.


5Operational Details of the Air Transportation System

The “GaTe” (a, B)

Most people have experienced operations at the gate. In commercial operations, the gate is where a commercial aircraft is parked to be serviced and load or offload passengers. Servicing includes being fueled; inspected; having perishable items, such as food and drinks, restocked; having the latrine serviced; and having cargo and/or passenger bags offloaded and onloaded. At the gate, prior to departure, pilots also perform preflight checks and prepare the aircraft for departure.

Private aircraft are typically not parked at a gate, but are parked in a spot on the ramp, and pas-sengers and crewmembers onload or offload through a door and stairs, and need to walk across the ramp to get to the terminal building. Often, at airports with both commercial and private service, this area and terminal building is a separate area from the commercial area.

Depending on the capabilities of the aircraft, at the gate pilots may need to load or check the flight plan into the aircraft’s flight management computer, if the aircraft is so equipped. (Most com-mercial aircraft do have such a system installed, whereas many older small aircraft do not.) The flight management computer, described in more detail in Chapter 2, interfaces with the autopilot system to help navigate the aircraft to its destination.

The efficiency of the servicing and onloading/offloading of passengers is important to com-mercial air carriers, as it determines the minimum amount of time required between an arrival of a flight and a departure of the same aircraft. In addition, passengers on commercial flights prefer not to sit awaiting departure any longer than they need to, so customer satisfaction with the commercial flight experience is somewhat dependent on the efficiency of this process. For that reason, research-ers and practitioners have studied this process and made recommendations for how it should be done (Bazargan, 2007; Hapsari, Clemes, & Dean, 2017; Kuo, 2014; Steffen, 2008; Steffen & Hotchkiss, 2012). Consensus on how to accomplish the passenger-loading process has not been achieved, as most passengers can attest, and is complicated by, among other things, airlines’ marketing deci-sions regarding providing preferential treatment to frequent fliers and other commercially important passengers. Passengers and crew on private or general aviation flights, in which the number of pas-sengers is very low or even zero, experience a much less-regulated process, loading whenever the aircraft is ready by walking out to the aircraft.

PushBack (c)

Pilots of aircraft flying under the control of air traffic controllers, that is, under instrument flight rules, must obtain a clearance from air traffic controllers before they are allowed to depart. Pilots flying under visual flight rules do not need a clearance. (See Chapter 3 for more information on the different types of flight rules and what they mean.) This clearance identifies for the pilots the route they must use to depart the general environs of the airport, that is, the departure to use along with the cleared route all the way to destination. An air traffic controller, referred to as clearance deliv-ery, typically delivers this clearance verbally over a radio frequency to the pilots, and the pilots read back the clearance to the controller to ensure they have recorded it correctly. Usually this clearance contains some instructions for immediately after departure and then just identifies any changes to the preplanned and prefiled flight plan the airline (or pilots) have sent in to the relevant air traffic control authorities.

Once pilots have obtained a clearance and all preflight operations, including passenger loading if applicable, have been completed, pilots of aircraft parked at a gate can request a pushback. As aircraft at a gate need to go backward, and can only go in reverse with great difficulty, a truck is used to push the aircraft back away from the gate.

This request, and the pushback, initiates the taxi portion of the flight, where the aircraft tran-sits from the parking area to the runway. Aircraft not parked at gate, or those who have already received a pushback away from the gate, then request permission from a ground or ramp controller to taxi.



Taxi OuT (D)

At controlled airports, permission is required to taxi, whereas at uncontrolled airports pilots do so on their own authority. Permission to taxi typically includes specific instructions about what taxi-ways to use and what to do when the aircraft gets to the runway.

During taxi, pilots are responsible for ensuring that they remain on the correct taxiways and stay clear of obstacles, vehicles, and other aircraft. At busy airports, markings visible from the pilot’s seat are provided to indicate taxiway identification, which is usually by letters such as taxi-way K, closed taxiways, taxiway centerline indications, and markings indicating the intersection of taxiways with runways. Sometimes, these markings are accompanied by a lighting system provid-ing these same indications but making them more visible at night or in poor visibility conditions. Uncontrolled airports typically have few or even no such markings, relying on pilots to navigate the taxiway system without assistance.

TakinG The Runway (e, F)

As indicated in the previous section, taxi instructions, when provided, usually include specifics on what the pilot should do when reaching the runway. Typically, this is an instruction for the pilot to hold short of the runway and contact the tower controller.

After contacting the tower controller, the pilot is given instructions to continue holding, to taxi on to the runway and hold their position, or is cleared to takeoff. This decision is dependent on whether an aircraft is still on, or about to land on, the runway, as only one aircraft is allowed to use the runway for takeoff or landing at any one time.

This decision may also reflect a desire to keep the departing aircraft from encountering the wake vortex of the aircraft that just departed. Wake vortices are not fully understood, as they are relatively complicated fluid dynamics phenomena, and have been modeled in a number of ways (Rossow, 2002; Shortle & Jeddi, 2007). However, as interactions with wake vortices are extremely dangerous and have caused fatal accidents, several minutes between departures (and arrivals) on the same runway are required when the aircraft involved are very heavy, such as large commercial aircraft, which can also affect runway capacity (Burnham, Hallock, & Greene, 2002).

TakeOFF (G)

Once cleared for takeoff, pilots advance power and accelerate to achieve a speed at which the air-craft can become airborne and fly safely. This process is actually one of the more dangerous parts of the flight, because, should a malfunction occur during the takeoff roll, pilots must make deter-minations as to whether it is safer to try to stop the aircraft on the runway or take the aircraft into the air with the malfunction. This is not a trivial decision, as either choice could lead to a serious, even fatal, accident.

This decision is referred to as the go–no go decision. At least for commercial aircraft, a speed is identified during preflight at which the aircraft must go regardless of the malfunction, short of the aircraft becoming nonairworthy. (All commercial aircraft are engineered such that they can fly with one engine not operating.) For large commercial aircraft, this speed is typically referred to as “V1,” which is short for “velocity #1.” This decision has been the subject of several academic papers, mostly on whether any automation assistance is possible in this case (Balachandran, Ozay, & Atkins, 2016; Bove & Andersen, 2002).

Another speed is identified at which the aircraft’s nose should be lifted so that the aircraft will lift off and fly off of the runway. This speed is referred to as “V2.”



aRRival GROunD OPeRaTiOns (s–v)

At controlled airports, the departure ground operations (gate, taxi, and takeoff) are reversed on arrival. Approaching aircraft contact the tower for approval to land; such approval includes specifi-cally for which runway the approval is given. Pilots are approved only for landing on that runway unless other instructions are given. After landing and before departing the runway, the aircraft is instructed to contact the ground controller on a separate radio frequency. At uncontrolled airports, pilots land and taxi on their own authority, visually clearing the area of any other aircraft or vehicles that would pose an obstruction or collision risk.

From there, again at controlled airports, the aircraft is given specific instructions to taxi to the parking area. At busy commercial airports, this includes a gate at which the aircraft is cleared to park. The gate assignment is typically given by the airline that controls that gate, although airlines can lend gates, although this is done rarely, if ever.

AIRBORNE OPERATIONS

After departure, aircraft talk to a series of air traffic controllers who manage volumes of airspace and control all aircraft within those volumes that are following instrument flight rules. See Chapters 3 and 15 for more information on airspace segmentation.

Initially, pilots talk to departure controllers, who are part of the terminal radio detection and ranging (RADAR) approach control facility, or TRACON. Close to controlled airports, within about 50 miles and up to about 10,000 feet, pilots talk to these departure controllers. The dashed box in Figure 1.3 is intended to depict the TRACON area of control, although the figure is not to scale. Pilots again talk to arrival controllers, also part of the TRACON, when approaching their destination.

The largest airport regions, about 27 of them, have TRACONS that are entirely separate from the control tower, although some, or even many, of these facilities are located in the basement

Departures

Arrivals

FIGURE 1.3 Atlanta arrival and departure routes (From Air Traffic Atlanta, http://www.airtrafficatlanta.com/channels.php? encid=8).


http://www.airtrafficatlanta.com/channels.php?encid=8

http://www.airtrafficatlanta.com/channels.php?encid=8


of the air traffic control tower building. The largest few regions, such as near Washington, D.C. and Los Angeles, have consolidated multiple TRACONs that control the arrival and departure traffic going into more than one airport. Such consolidated TRACONs are located in a separate building away from any of the airports they control.

For smaller airports, the tower controllers perform the arrival and departure control responsibili-ties. It is possible, in regions of the country where TRACON airspaces are contiguous, for aircraft to fly between airports only talking to tower controllers. Such operations are referred to as tower enroute operations.

No TRACON services are provided for uncontrolled airports. In such cases, pilots typically announce their position and intentions on a common frequency so that any other aircraft in the vicinity will know where other aircraft are by means other than visually acquiring them.

climBOuT (h–k)

Climbout is the portion of flight that transitions the aircraft from the immediate vicinity of the airport to high-altitude cruise flight. For small general aviation aircraft, high-altitude may be a few thousand feet; for commercial aircraft, cruise flight typically occurs between 29,000 and 41,000 feet. (Some small business jets can fly at slightly higher altitude, and the now-defunct super-sonic Concorde would cruise at up to 60,000 feet.)

Pilots are expected to follow the instructions given to them as part of their clearance, unless departure controllers give them different instructions, which they typically do. These instructions are designed to expedite the aircraft’s climb while keeping aircraft in the departure/arrival area safely separated from one another.

At busy airports, this departure process is very challenging for both pilots and controllers, as it involves separating climbing/accelerating and descending/decelerating aircraft, who are trying to go in roughly opposite directions. The problem is somewhat simplified procedurally by hav-ing arrival and departure aircraft fly along different corridors, which limits where these aircraft routes intersect. An example, for the Atlanta TRACON, is shown in Figure 1.3. As can be seen in Figure 1.3, however, these arrival and departure routes are very complex; aircraft are often given numerous climb, level-off, and heading instructions.

Further complicating matters, controllers who are providing approval for aircraft to climb can only estimate their climb performance. Although experience provides them with some guidance, unknown weights and idiosyncrasies among pilots make this estimate error-prone. Controllers must be conservative in allowing departures to climb, as it is difficult to be sure that a climbing aircraft will reach a certain altitude prior to the intersection of that departing aircraft’s flight path with that of an arriving aircraft. Departing aircraft are, therefore, often given intermediate level-off altitudes, as shown in Figure 1.2 with letter J, which makes the climbout less efficient, and which results in less cruise time, where the aircraft is the most fuel efficient.

Climbout is considered one of the higher workload periods for pilots of large commercial aircraft. Pilots experience a large number of communications during this period, must make many changes to the flight path of the aircraft either manually or to the automation that is controlling the aircraft, and must remain vigilant due to the high density of aircraft in the area.

cRuise (l)

After leaving the TRACON, aircraft flying under instrument flight rules are handed off to the first of a sequence of enroute controllers. These controllers sit in a facility that is separate physically from TRACONs and/or towers. These Air Route Traffic Control Centers control large volumes of airspace, in which the Center volume is broken up into irregular volumes of airspace called sectors. Groups of sectors on which individual controllers are trained are called areas.



Sectors are shaped to conform to the typical flows of aircraft that transit the sectors. Some sectors primarily handle arrival and/or departure traffic to/from TRACONS and are oriented accordingly – typically long and slender in the predominant direction of flight for the arriving/departing aircraft. Sectors that handle cruise aircraft in which traffic density is low are typically large and shaped more regularly, such as a large box shape.

Above 20,000 feet where most large commercial aircraft fly, aircraft at cruise are separated into flight levels (FL) corresponding to the altitude in hundreds of feet, such that aircraft flying gener-ally eastbound (0°–179°) are required to fly at odd FL(FL210 or 21,000 feet, FL230, FL250, FL270, FL290, FL330, FL370, etc.), and aircraft flying westbound are required to fly at even FL (FL220, FL240, FL260, FL280, FL310, FL350, etc.). These rules vary internationally, including FL being defined by meters instead of feet in some Asian and Eastern European countries.

The 2,000 versus 1,000-feet change in the difference between eastbound and westbound alti-tudes the reader might note above FL290 is reduced to a 1,000-feet difference in areas in which reduced vertical separation minima airspace has been introduced. Only certain properly equipped aircraft are allowed to use these altitudes in that airspace.

Cruise flight can comprise the majority of the time a flight is airborne. It is generally considered a low-risk flight regime, due to little climbing and descending and long periods in low- density airspace. It is also considered low-workload, as for properly equipped aircraft the autopilot is typically engaged, often coupled to the flight management system, leaving pilots the task of com-municating infrequently with air traffic controllers and monitoring the autopilot and other systems.

DescenT (m, n, O)

When getting within a few hundred miles of their destination, depending on cruise altitude, aircraft may begin to descend. This descent may be initiated by controllers or be initiated by the pilots. A typical descent profile consists of a set of descents to lower altitudes, a level-off for a period of time, followed by another descent. This is referred to as a staircase type descent because of its resemblance to a staircase.

The level-off portions of such descents are inefficient, as pilots must increase power to keep the aircraft level. Air traffic authorities and researchers have been investigating, and in some cases even implementing, continuous descent procedures for commercial operations that elimi-nates these inefficiencies (Clarke et al., 2004; Coppenbarger, Mead, & Sweet, 2007; Dinges, 2007; Green & Vivona, 2001; Landry, 2009; Robinson & Kamgarpour, 2010; Staigle & Nagle, 2006; Tong, Boyle, & Warren, 2006; White & Clarke, 2006). Such procedures have the aircraft cruise to a point in which they can continuously descent, at near idle power, to very close to runway touch-down, thereby eliminating the inefficient level-off periods. Of course, such aircraft still have to be separated from other arriving and departing aircraft, making the operation challenging to execute. As of this writing, such operations are limited to low-demand periods or to airports in which the operations can be carefully controlled.

Descent again begins a higher workload period, in which there is typically a higher communica-tion workload and more maneuvering. This period is also more risky than cruise flight, due to more complex interactions with other climbing, descending, and cruising aircraft.

aRRival (P, Q)

Workload and risk increases further as the aircraft gets closer to the airport. Center controllers descend the aircraft and deliver it to a point of entrance (a fix) to the TRACON at a particular altitude based on agreements made between the relevant Center and TRACON. Center control-lers hand off the aircraft to the arriving TRACON, in which aircraft talk to an arrival controller. Different TRACONS handle incoming aircraft differently, either by having a set of arrival sectors



much similar to those found in Center airspace, or by having one or more arrival controllers that handle aircraft from TRACON entry to Tower handoff.

Arrival controllers can refuse to accept handoffs from Center controllers if the TRACON is too busy. In such cases, aircraft must then enter a holding pattern, which is a racetrack pattern consist-ing of straightaway legs of 10 or 20 nautical miles.

If the TRACON controller continues to refuse aircraft entry, subsequent aircraft are stacked in holding patterns at altitudes above the first. When the Center controller runs out of vertical space, aircraft are held further out from the TRACON, and so on. In particularly egregious situations, such holding aircraft can even block departure routes from the (arriving) runway, resulting in gridlock in which departing aircraft cannot leave their gates due to blocked airspace and arriving aircraft cannot arrive due to blocked gates. This is fortunately rare.

Aircraft under instrument flight rules are most commonly given headings (vectors) and altitudes to fly to get aligned for landing. However, aircraft could also be cleared for a particular arrival pro-cedure, which requires them to fly a particular set of route legs, speeds, and altitudes. Aircraft are expected to eventually take over navigating to the runway, either by visually acquiring the runway and positioning themselves to land, or by following ground-based navigation systems such as an instrument landing system that provides both lateral and vertical (glidepath) guidance.

Smaller, general aviation aircraft must find and align themselves to land based on whatever equipment the aircraft possesses, which may be very little. Most commonly, such aircraft use visual references to find and land at the airport.

lanDinG anD GO-aROunD (R)

Using such guidance, aircraft can position themselves to land, obtaining permission to do so from the tower, if the airport is controlled. However, if the aircraft for some reason cannot land safely, such as not having sufficient visual references to identify the runway environment, they must execute a go-around in which the aircraft climbs away from the runway and back into the approach environ-ment. Other reasons for go-arounds include vehicles/aircraft on the runway and aircraft malfunc-tions. There is commonly a designated procedure for go-arounds that aircraft are expected to follow, not only to ensure predictability but also to ensure safe separation from ground obstructions.

Go-arounds are very risky and high workload periods. Go-arounds are not executed very often, so pilots are initiating a procedure with which they are not entirely familiar. There is also high demand on them, with configuration changes (landing gear, flaps) occurring, communication requirements, and navigation requirements. Moreover, most go-arounds are flown manually, exac-erbating the workload issue.

CONCLUSION

Aircraft operations are fairly complex, with a great deal of information that can be difficult to find if one does not have actual operational experience. Hopefully, the current chapter has pro-vided an overview of how aircraft move through the air transportation system. With reference to the more detailed and comprehensive information in the subsequent chapters, readers should be able to become sufficiently knowledgeable about the system to be productive and effective researchers and practitioners.

REFERENCES

Balachandran, S., Ozay, N., & Atkins, E. M. (2016). Verification guided refinement of flight safety assessment and management system for takeoff. Journal of Aerospace Computing, Information and Communication, 13, 357–369.

Bazargan, M. (2007). A linear programming approach for aircraft boarding strategy. European Journal of Operational Research, 183, 394–411.



Bove, T., & Andersen, H. B. (2002). The effect of an advisory system on pilots’ go/no-go decision during take-off. Reliability Engineering and System Safety, 75, 179–191.

Burnham, D. C., Hallock, J. N., & Greene, G. C. (2002). Wake turbulence limits on paired approaches to parallel runways. Journal of Aircraft, 39, 630–637.

Clarke, J.-P., Ho, N. T., Ren, L., Brown, J. A., Elmer, K. R., Zou, K., … Warren, A. W. (2004). Continuous descent approach: Design and flight test for Louisville International Airport. Journal of Aircraft, 41, 1054–1066.

Coppenbarger, R., Mead, R., & Sweet, D. (2007, September). Field evaluation of the tailored arrivals concept for datalink-enabled continuous descent approach. Paper presented at the 7th AIAA aviation technol-ogy, integration, and operations conference, Belfast, Northern Ireland.

Dinges, E. (2007, July). Determining the environmental benefits of implementing continuous descent arrival operations at Atlanta and Miami (paper no. 594). Paper presented at the 7th USA/Europe seminar on air traffic management research and development, Barcelona, Spain.

Federal Aviation Administration. (2016). Aeronautical information manual: Official guide to basic flight information and ATC procedures, Washington, DC: U.S. Department of Transportation, Federal Aviation Administration.

Green, S. M., & Vivona, R. A. (2001, August). En route descent advisor concept for arrival metering. Paper presented at the AIAA guidance, navigation, and control conference, Montreal, Canada.

Hapsari, R., Clemes, M. D., & Dean, D. (2017). The impact of service quality and customer engagement and marketing constructs on airline passenger loyalty. International Journal of Quality and Service Sciences, 9, 2–29.

Kuo, C.-C. (2014). A review of heuristic and optimal aircraft boarding strategies in the U.S. airline industry. In M. A. Warkentin (Ed.), Trends and research in the decision sciences, pp. 313–330. Upper Saddle River, NJ: Decision Sciences Institute.

Landry, S. J. (2009). Mid-term CDA operations to ATL: Delay requirements under nominal and off-nominal conditions. Paper presented at the NASA airspace technical interchange Meeting, San Antonio, TX.

Robinson III, J. E., & Kamgarpour, M. (2010, September ). Benefits of continuous descent operations in high-density terminal airspace under scheduling constraints. Paper presented at the 10th AIAA aviation technology, integration, and operations conference, Fort Worth, TX.

Rossow, V. J. (2002). Reduction of uncertainties in prediction of wake vortex locations. Journal of Aircraft, 39, 587–596.

Shortle, J., & Jeddi, B. (2007). Using multilateration data in probabilistic analysis of wake vortex hazards for landing aircraft. Transportation Research Record: Journal of the Transportation Research Board, 2007, 90–96.

Staigle, T., & Nagle, G. (2006, September). Details and status of CDA procedures for early morning arrivals at Hartsfield-Jackson Atlanta International Airport (ATL). Paper presented at the CDA Workshop No. 3, Atlanta, GA.

Steffen, J. H. (2008). Optimal boarding method for airline passengers. Journal of Air Transport Management, 14, 146–150.

Steffen, J. H., & Hotchkiss, J. (2012). Experimental test of airplane boarding methods. Journal of Air Transport Management, 18, 64–67.

Tong, K.-O., Boyle, D. A., & Warren, A. W. (2006, September). Development of continuous descent arrival (CDA) procedures for dual-runway operations at Houston Intercontinental. Paper presented at the 6th AIAA aviation technology, integration, and operations conference, Wichita, KS.

White, W., & Clarke, J.-P. B. (2006, September). Details and status of CDA procedures at Los Angeles international airport (LAX). Paper presented at the CDA workshop No. 3, Atlanta, GA.


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1 Operational Details of the Air Transportation System · 4 Handbook of Human Factors in Air Transportation Systems GROUND OPERATIONS Controlled airports, where air transportation