## Design of Taxiway and Exit Taxiway in Airport Engineering

Design of Taxiway and Exit Taxiway in Airport Engineering Step onto the tarmac of airport engineering as we navigate through the intricacies of taxiway and exit taxiway design. In this blog post, we unfold the blueprint of success with a focus on essential principles and questions that will guide you through mastering the art of aviation infrastructure.

## introduction

### what is taxiway?

**Taxiway:** The path which provides aircraft to access the runway and the loading apron or the service hanger back and forth is called the taxiway.

### Key aspects to be kept in mind while designing taxiway:

i) The aircraft which has just landed and is moving towards the apron should not interfere with the aircraft taxing for take-off.

ii) **Exit taxiway:** For smooth functioning at a busy airport, the landing aircraft should leave the runway as early as possible, thereby keeping the runway clear for use by other aircraft.

iii) Shortest practicable distance from the apron to the runway end.

iv) Intersections of taxiways should be avoided.

v) Exit taxiway should be designed for high turn off speeds. vi) Reduce the runway occupancy time of aircraft.

### parameter is taken in consideration for geometric design of taxiway:

### recommendations:

**International Civil Aviation Organisations Recommendations**

**Longitudinal gradient**

ICAO recommends that the longitudinal gradient should not exceed 1.5% for A and 8 types and 3% for other types of airports.

**Transverse gradient**

ICAO recommends that for taxiways, pavement like runways, the transverse gradient should not exceed a value of 1.5% for A, 8 and C and 2% for D and E type of airports.

**Rate of change of slope**

ICAO recommends that the rate of change of slope in longitudinal direction should not exceed 1% per 30 m length of the vertical curve for A and 8 and C and 1.2% for D and E type of airports.

**Visibility**

ICAO recommends that the surface of a taxiway must be visible from 3 m height for a distance of 300 m for A, 8 and C types and a distance of 250

## TURNING RADIUS

• A horizontal curve is provided whenever there is a change in the direction of a taxiway.

• These horizontal curves are so designed that aircraft do not have any significant speed reduction while negotiating for the curve.

• The radius can be obtained from the following formula

**R = V ^{2}/125f**

R: Radius of the curve in meter

V: Speed in kmph

f: Coefficient of friction between tyre and pavement (if not given, assume f = 0.13)

• Minimum radius of the curve is 120 m for large subsonic aircrafts, and it is 180 m for supersonic transports aircrafts.

• According to Horonjeff's equation, the radius of the taxiways should be provided such that the distance of the oleo strut of the nearby main gear is not less than 6 m from the pavement edge.

• The relationship between the radius of the taxiway, the wheelbase of the aircraft and the specified distance of the main gear from the edge of the pavement is given by the following equation given by Horonjeff

**R= 0.388W ^{2}/(T/2)-S**

Where R = Radius of taxiway in meter

W = Wheelbase of aircraft in meter

T = Width of taxiway pavement in meter

S = Distance between the midway point of the main gears and the edge of the taxiway pavement in metres.

• If the pilot maintains the nose gear on the centre line of the taxiway, having the radius as obtained from the above expression, the oleo strut of the main gear of the aircraft would not come closer than 6m from pavement edge.

## important questions

### Q 1) A taxiway is to be designed for Boeing 792-215 (Subsonic aircraft).Following data are given in reference to the aircraft, Wheelbase length: 17.80m, Tread of the main landing gear: 6.72m, Turning speed of aircraft: 42 kmph, Coefficient of friction between type and the pavement surface: 0.13

**Sol:** **i)** Turning radius R = V^{2}/ 125f

= (42)^{2}/125x0.13

= 108.56m

**ii)** From Horonjeff's equation,

R = 0.388W^{2}/(T/2-S)

W = 17.80 m

T = 22.5 m

S = 6 + 6.72/2 = 9.36 m

Substituting the values,

R = 0.388 x (17.80)^{2} / 11.25 -9.36 =65.05m

**iii) **The absolute minimum turning radius required by a subsonic aircraft, regardless of its speed is 120m. Selecting the maximum value amongst the above three values, the turning radius provided for the taxiway is 120m.

### Q 2) Which of the following is/are correct?

### I) Route for the taxiway should be selected such that it provides the shortest feasible distance from the apron to the runway end.

### II) As far as the possible intersection of the taxiway and runway should be avoided.

a) i) only

b) i) only

c) i) and ii) both

d) Neither i) nor ii)

**Sol:** Both the statements are correct considerations regarding the taxiway layout. Taxiway route should be selected such that it provides the shortest feasible distance from the apron to runway end. As far as the possible intersection of the taxiway and runway should be avoided.

So, the correct option is **c)**

**Q 3)** A taxiway is to be designed for an aircraft having the following characteristics Wheelbase 18 m Tread of main landing gear 6.85 m Turning speed 48 kmph What will be the turning radius as per Horonjeff's equation?

**Sol:** R = 0.388W^{2} / (T/2) - S

W = 18 m, T = 22.5 m

S = 6 + 6.85/2 = 9.425 m

R = (0.388 * 18^{2})/(22.5/2) - 9.425 = 68.88 m

**Q 4)** The design speed of the exit taxiway is 80 kmph, and the coefficient friction is 0.165. The radius of the exit taxiway ...m?

**Sol:** R = V^{2}/125f

R = 80^{2}/125 * 0.165

R = 310.3 m

**Q 5)** The ratio of the minimum turning radius for a subsonic aircraft to that of supersonic aircraft is:

a) 1.5

c) 2

b) 0.67

d) 0.5

**Sol:** Minimum radius required for supersonic aircraft = 180 m

Minimum radius required for subsonic aircraft = 120 m

Ratio = R_{subsonic/ }R_{supersonic}

_{ }= 120/180 = 0.67

Correct answer is **b)**

**EXIT TAXIWAY**

### The location of the exit taxiway depends on:

- Number of exit taxiways
- Pilot variability
- Type of aircraft
- Topographical features
- Exit Speed
- Weather condition

**Description of factors:**

**Description of factors**

**1) Number of exit taxiway**

The location of the exit taxiway is decided by their number.

Exit taxiways are generally provided at the runway ends if taxiways are two in number.

**2) Exit speed**

The maximum exit speed by which an aircraft can exit or enter the exit taxiway is restricted for each aircraft. A particular stretch of the runway is required by aircraft to decelerate to turnoff speed.

**3) Type of aircraft**

Speed of landing depends on the type of aircraft; hence exit speed differs with the type of aircraft performing landing operations.

**4) Weather condition**

Wind temperature, fog etc., affect the landing speed of aircraft. This also affects the distance required by the aircraft to slow down to the exit speed.

**5) Topographical features**

The high altitude or deep valley reduces visibility, which may affect the landing speed. Obstruction in approach and turning zones may also influence the landing speed and hence affect the location of the exit taxiway.

**6) Pilot variability**

The rules for the landing of transport category aircraft are quite precise. Even then, some variability amongst different pilots does occur, especially in the distance from the runway threshold to the touchdown point and the application of brakes on the runway.

**Design of exit taxiway connecting runway and parallel taxiway**

**The following concept governs the design of the taxiway.**

- Turning radius
- Smaller angle
- Widened entrance
- Compound curve

**i)** Exit speed of aircraft plays a vital role in deciding turning radius.

**ii)** An entrance with a width of 30m, along with a gradual narrowing to the normal width of the taxiway is preferred.

**iii)** The smaller angle appears to be desirable as the length of the curved path is decreased, so a total angle of turn of 30° - 45° can be negotiated effectively.

**iv) **A compound curve is crucial to reduce the wearing of tyres on the nose gear. Thus, the larger radius curve R, should succeed the main curve radius R, as shown below. A spiral path may be adopted. Although a compound curve (as its shape is similar to a spiral curve) is chosen as it is comparatively easier to construct in the field.

The following radius was found experimentally suitable.

Speed (kmph) | Radius (m) |

65 | 517 |

80 | 731 |

95 | 941 |

**Speed and Radius**

R_{1} = radius of entrance curve

L_{1} = length of entrance curve

R_{2} = radius of central curve

L_{2} = length of central curve

**v) **The length of the larger radius curve can be roughly obtained from the following relation.

L=(0.28V)^{3}/CR_{2 }= V^{3}/45.5 x C x R_{2}

The value of C is 0.39.

**vi) **Sufficient distance must be provided to easily decelerate an aircraft after it leaves the runway. This distance is based on an average deceleration rate of 1 m/sec? (3.3 ft/sec?). The stopping distance may be obtained from the following equation

S.D.= (0.28V^{2})/2d = V^{2 }/ 25.5d

where d is the deceleration in m/sec^{2}. The stopping distance must be measured from the edge of the runway pavement along the exit taxiway.

## Important question

**Q 1)** The turnoff speed and total angle of turn are 80 kmph and 30° respectively. Design and sketch an exit taxiway joining a runway and a taxiway.

**Sol:** The various design elements of the exit taxiway are shown in the figure below:

Radius of the central curve

R_{2} = V^{2}/125f = (80)^{2}/125*0.13 = 391 m

The radius of the entrance curve R, is obtained from the following table.

Speed (kmph) | 65 | 80 | 95 |

Radius (R_{1}) m | 517 | 731 | 941 |

Therefore, provide R, = 731 m

Length of the entrance curve is given by

Length of the entrance curve is given by

L_{1} = V^{3}_{1} / 45.5 CR_{2} = (80)^{3} / 45.5 * 0.39 * 391 = 73.5 m

Defection angle of the central curve

Δ_{1} = 180 L_{1} / π R_{1} = 180 * 73.5 / 3.14 * 731 = 5.75° = 5°45'

Deflection angle of the central curve

Δ_{2} = 30° - 5°45' = 24°15'

Length of the central curve

L_{2} = πR_{2}Δ_{2} / 180 =3.14 * 391 * 24.25 / 180 = 165.5 m

Stopping Distance S.D = V^{2} / 25.50d

Assuming the deceleration rate 1 m/sec^{2}

S.D = (80)^{2} / 25.50 * 1 = 251.0 m

• This distance will be measured from the edge of the runway pavement along the centre line of the exit taxiway. Assuming a major airport installation with instrumental landing facilities, the separation clearance

as specified by ICAO = 198.70m.

• Therefore, the available length of the exit taxiway, assuming the width of the runway and main taxiway as 45 m and 22.5 m, respectively, can be obtained as,

= 198.70/sin 30°= [45 + 22.5/2 sin 30°] = 397.40-67.5 = 329.9m.

• It should now be checked that this distance is at least equal to the stopping distance as calculated above. Sometimes the separation clearance distance may have to be increased to accommodate the stopping distance within the length of the exit taxiway.

• Fillet's radii 22.5 m and 60 m are provided for 30° and 150° changes in the directions, respectively.

## gate capacity

• Space adjacent to a terminal building used for parking of an aircraft for loading and unloading of passengers, baggage and mail are referred to as GATE.

### What is GATE CAPACITY?

**Sol:** **Gate Capacity:** The capability of a particular number of gates to accommodate aircraft for loading and unloading operations under the prevailing condition of continuous demand is termed as gate capacity.

Gate capacity is the inverse of the weighted average gate occupancy time for all the aircraft served.

**Factor affecting gate occupancy**

- Type of aircraft
- The amount of baggage and mails
- Originating, turn around, or through flight
- The efficiency of apron personnel
- Boarding & deboarding of passengers
- Availability of gates for all user or exclusive for one airline or aircraft

## Important Question

### Q 1) What is the capacity of 12 gates that serve three classes of aircraft using the following aircraft mix and average gate occupancy time?

Aircraft Class | Mix (%) | Average Occupancy Time (min) |

1 | 15 | 25 |

2 | 35 | 45 |

3 | 50 | 60 |

**Sol:** Considering that each gate is available for all aircraft.

Gates capacity for a single gate is given by

C_{sg} = 1 / weighted service time

= 1 / (0.15 × 25) + (0.35 × 45) + (0.5 × 60)

= 0.02020 aircraft / min / gate

If G = the total number of gates, the capacity for all gates is

C= G. Cs = 10x0.02020 = 0.202 aircraft / min or 12.12 aircraft/hr ~ 12 aircraft/hr

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