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Airport slots

Reading & Requesting
28 May 2025 by
Geoffray Lamarsalle

Introduction

The growth of global air traffic has led to congestion phenomena at airports. For more effective regulation, the busiest airports have implemented a slot system. A slot is a movement authorisation at an airport allocated for a flight at a specified date and time..

Expressing concern over the increasingly large queues of aircraft both on the ground and in approach, leading to delays and unnecessary fuel consumption, the FAA established its first slot system as early as 1968 at the four major airports in the United States (New York J.F. Kennedy, La Guardia, Washington National, and Chicago O'Hare), and it gradually expanded to other airports. Europe adopted a similar regulation in 1993, and other regions of the world followed suit. Today, 43% of civil airports require the allocation of slots to operate from their facilities.. 


Slots allocation

Each country has a slot coordination unit that defines the number of slots for each airport based on their capacity.

The slots are available in limited numbers and are reallocated for each IATA season among operators during a biannual conference (IATA Slot Conference). They are required to use at least 80% of the approved slots in order to renew them for the following season. This is known as the "grandfather right" rule, which particularly ensures that the largest historical operators can renew their traffic rights. Unused or newly created slots can be allocated to new applicants.

Some periods or hours of operation may be subject to a reduced number of slots, as is the case for night flights, for example..


Classification of airports

When an airport imposes the allocation of slots, it is referred to as a "coordinated" airport. IATA classifies them into 3 levels according to the degree of congestion. :

  • Niveau 1 : Not coordinated
  • Niveau 2 : Facilitated Airport Schedules. Not coordinated but presenting a risk of congestion at certain times. Schedule adjustments are agreed upon by mutual consent with the airlines. 
  • Niveau 3 : Coordinated


Message structure

Flights operating to level 3 airports must therefore obtain slots to operate there, but also be able to modify or cancel them if necessary..

To this end, a protocol has been established in the IATA SSIM (Standard Schedules Information Manual) defining the procedures and format for request messages. These were originally telegram messages but are now sent and received by email. There are two main types of these messages.:

  • SCR (Slot Clearance Request)
  • SIR (Slot Information Request)

The message is divided into 3 parts:

​1. Header

SCR                                       > type of message: SCR ou SIR
/ops@avionops.com        > sender email
S25                                       > IATA season on date of flight
27MAY                                 > date of message
BRU                                       > IATA airport code

2. Data

N AV123 31MAY 0000060 262332 1000BRU J

 Operator and coordinator responses

FLIGHT NUMBER

    • Format IATA XX123

DATE

    • Format DDMMM

DAY OF THE WEEK

    • Position of the day operated from 1 to 7
    • 0 for non-operated days

AICRAFT TYPE

    • Nombre de siège / Type d’avion au format IATA 123123

AIRPORT

    • Airport code in IATA format XXX

SCHEDULED TIME

    • Seat number / Aircraft type in IATA format 123123

TYPE OF FLIGHT

 Type of service

​3. Footer

SI: Additional information (special requests, clarifications,…)
GI : General information (greetings, signature,…)

Example of standard message:

It is 27th May and we would like to request a slot for a departure from Brussels National Airport (BRU) :

    • Flight N° : AV123
    • ETD : Saturday 31 May /1000UTC
    • Aircraft : Airbus A330-200 with 262 seats
    • Destination : Dubai International (DXB)
    • Type of flight: vol charter PAX

We therefore need to request to send an SCR to the Brussels coordination:

SCR
/ops@avionops.com
S25
27MAY
BRU
N AV123 31MAY 0000060 262332 1000DXB C
SI IF NOT AVBL PLS GIVE NEAREST POSSIBLE
GI BRGDS AVN OPS

 

SCR (Slot Clearances Request) sent by the operatorr

Here are examples of messages depending on the type of request addressed to the coordinator of Brussels Airport:

New flight - Departure

Departure to Dubai at 1000 UTC on Saturday 31 May :

SCR
/ops@avionops.com
S25
27MAY
BRU
N  AV124 31MAY 0000060 262332 1000DXB C
SI
GI BRGDS AVN OPS
    • Put a space between the action code N (new) and the flight number.
    • Indicate the day of the week.
    • The destination is located after the ETD.

New Flight - Arrival

Arrival from Dubai at 0700 UTC on Saturday 31 May :

SCR
/ops@avionops.com
S25
27MAY
BRU
NAV123 31MAY 0000060 262332 DXB0700 C
SI
GI BRGDS AVN OPS
    • No space between the action code N (new) and the flight number.
    • The destination is before the ETA.

Nouveau vol - Turnaround ad-hoc

Arrival from Dubai at 0700UTC and departure to Dubai at 1000UTC on Saturday 31 May :

SCR
/ops@avionops.com
S25
27MAY
BRU
NKF124 KF123 31MAY31MAY 0000060 262332 DXB0700 1000DXB CC
SI
GI BRGDS AVN OPS
    • The flight numbers, date, and ETD/ETA are on the same line.
    • The type of flight must be indicated twice.

New flight - Turnaround by period

Arrival from Dubai at 0700UTC and departure to DXB at 1000UTC from 26 May to 01 June on Tuesday, Wednesday and Saturday :

SCR
/ops@avionops.com
S25
27MAY
BRU
NKF124 KF123 26MAY02MAY 0230060 262332 DXB0700 1000DXB CC
SI
GI BRGDS AVN OPS
    • The date is the desired operating period.
    • Include the days of the week of operation.

Change

Cchange of aircraft type (A330-300 with 286 seats instead of A330-200 with 262 seats) and ETD (1400UTC instead of 1000z) :

SCR
/ops@avionops.com
S25
27MAY
BRU
CKF124 KF123 26MAY02MAY 0240060 262332 DXB0700 1000DXB CC
RKF124 KF123 26MAY02MAY 0240060 286333 DXB0700 1400DXB CC
SI DELAY AND ACFT CHG FOR TECHNICAL REASON
GI BRGDS AVN OPS
    • Put “C” (change) in front of the line to be changed and “R” (revise) in front of the new line.
    • Indicate the new type of aircraft.

Cancellation

Cancellation of Tuesday's flight only:

SCR
/ops@avionops.com
S25
27MAY
BRU
DF124 KF123 27MAY27MAY 0200000 262332 DXB0700 1000DXB CC
SI CANCELLATION FOR OPERATIONNAL REASON
GI BRGDS AVN OPS
    • Put a D (delete) in front of the vo numberl.
    • Indicate the date of the flight and the day of the week of the flight to be cancelled.

Acceptance of an offer

The coordination suggests a departure at 1015z instead of the requested 1000UTC :

SCR
/ops@avionops.com
S25
27MAY
BRU
A  AV124 31MAY 0000060 262332 1015DXB C
SI
GI BRGDS AVN OPS
    •  Put an A in front of the flight number to accept the proposed time.

Acceptance of an offer with maintain on the waiting list

The coordination is proposing a departure at 1015UTC instead of the requested 1000UTC. However, we would prefer to stick to 1000z if a slot becomes available. :

SCR
/ops@avionops.com
S25
27MAY
BRU
P  AV124 31MAY 0000060 262332 1015DXB C
SI PLEASE ADVISE IF NEW OFFER AVAILABLE
GI BRGDS AVN OPS
    •  Put a P in front of the flight number to accept the proposed time while remaining on the waiting list.

Decline of an offer

The coordination is proposing a departure at 1015UTC instead of the requested 1000UTC. We decline. :

SCR
/ops@avionops.com
S25
27MAY
BRU
Z  AV124 31MAY 0000060 262332 1015DXB C
SI OFFER NOT ACCEPTABLE DUE DESTINATION AIRPORT CURFEW
GI BRGDS AVN OPS
    •  Put a Z in front of the flight number to decline.


SCR (Slot Clearances Request) sent by the coordinator

Below are some standard responses from the coordinator:

Confirmation

Our request for an arrival from Dubai at 0700 UTC and departure to Dubai at 1000 UTC on Saturday 31 May is accepted :

SCR
/ops@avionops.com
S25
27MAY
BRU
KKF124 KF123 31MAY31MAY 0000060 262332 DXB0700 1000DXB CC
SI
GI BRGDS AVN OPS
    •  The answer starts with a K.

Cancellation

The flight cancellation is acknowledged :

SCR
/ops@avionops.com
S25
27MAY
BRU
XF124 KF123 27MAY27MAY 0200000 262332 DXB0700 1000DXB CC
SI
GI BRGDS AVN OPS
    •  ​The answer starts with an X.

Change

Our request for change has been accepted :

SCR
/ops@avionops.com
S25
27MAY
BRU
XKF124 KF123 26MAY02MAY 0240060 262332 DXB0700 1000DXB CC
KKF124 KF123 26MAY02MAY 0240060 286333 DXB0700 1400DXB CC
SI
GI BRGDS AVN OPS
    • The line to be modified starts with an X.
    • The new accepted request starts with a K.

New flight - Declined with new proposal

Our request for departure at 1000UTC has been denied, but the coordination team is offering us 1015UTC as an alternative: 

SCR
/ops@avionops.com
S25
27MAY
BRU
UKF124 KF123 31MAY31MAY 0000060 262332 DXB0700 1000DXB CC
OKF124 KF123 31MAY31MAY 0000060 262332 DXB0700 1015DXB CC
SI CLOSEST AVAILABLE OFFER
GI BRGDS AVN OPS
    • The rejected request starts with a U.
    • The new proposal starts with an O.

Change – Declined with new proposal

Our request to change the departure time from 1000UTC to 1400UTC has been refused. Coordination has proposed a departure at 1500UTC. However, the change of aircraft type has been duly noted...  

SCR
/ops@avionops.com
S25
27MAY
BRU
HKF124 KF123 26MAY02MAY 0240060 262332 DXB0700 1000DXB CC
UKF124 KF123 26MAY02MAY 0240060 286333 DXB0700 1400DXB CC
OKF124 KF123 26MAY02MAY 0240060 286333 DXB0700 1500DXB CC
SI CLOSEST AVAILABLE OFFER
GI BRGDS AVN OPS
    • The flight originally confirmed starts with an H.
    • The rejected request starts with a U.
    • The new proposal starts with an O.

New flight – Declined without proposal

The requested flight is refused by the coordination. No alternative proposal:

SCR
/ops@avionops.com
S25
27MAY
BRU
UKF124 KF123 31MAY31MAY 0000060 262332 DXB0700 1000DXB CC
SI UNABLE DUE TO RUNWAY CLOSURE
GI
    •  The rejected request starts with a U.

Flight not found

The flight to be cancelled or changed is not found in the coordination database :

SCR
/ops@avionops.com
S25
27MAY
BRU
WF124 KF123 27MAY27MAY 0200000 262332 DXB0700 1000DXB CC
SI UNABLE TO RECONCILE FLT INFROMATION
GI BRGDS AVN OPS
    •  The answer starts with a W.

 

SIR (Slot Information Request) sent by the operator

To check the status of the flights currently confirmed by the coordination, the operator sends a SIR :

Departure on a specific date

We want to verify the confirmation of flight AV123 on May 28 :

SIR
/ops@avionops.com
S25
27MAY
BRU
Q  AV123 28MAY
SI
GI BRGDS BELGIUM SLOT COORDINATION
    • Put a space between the action code Q (query) and the indicator.

Arrival on a specific date

We want to verify the confirmation of flight AV124 on May 28 :

SIR
/ops@avionops.com
S25
27MAY
BRU
QAV124 28MAY
SI
GI BRGDS BELGIUM SLOT COORDINATION
    •  No space between the action code Q and the indicator.

Turnaround on a specific date

We want to verify the confirmation of flights AV123 and AV124 on May 28 :

SIR
/ops@avionops.com
S25
27MAY
BRU
QAV124  AV123 28MAY
SI
GI BRGDS BELGIUM SLOT COORDINATION
    • Put a space between the two flight numbers.

All flights over the same period in turnaround format

We want to verify the list of all confirmed movements in Brussels from June 1 to June 7 :

SIR
/ops@avionops.com
S25
27MAY
BRU
QAV  AV 01JUN07JUN
SI
GI BRGDS BELGIUM SLOT COORDINATION
    • Put the operator's indicator twice with a space between the two.

Departures only during the same period​

We want the list of all confirmed departures to Brussels from June 1 to June 7 :

SIR
/ops@avionops.com
S25
27MAY
BRU
Q  AV 01JUN07JUN
SI
GI BRGDS BELGIUM SLOT COORDINATION
    • For departures, put a space between the action code Q and the indicator.

Arrival only within the same period

We want the list of all confirmed arrivals in Brussels from 1st to 7th June :

SIR
/ops@avionops.com
S25
27MAY
BRU
QAV 01JUN07JUN
SI
GI BRGDS BELGIUM SLOT COORDINATION
    • For arrivals, there should be no space between the action code and the indicator.

Departures and arrivals separately

We want the list of all confirmed arrivals and departures in Brussels from 1st to 7th June :

SIR
/ops@avionops.com
S25
27MAY
BRU
QAV 01JUN07JUN
Q  KF 01JUN07JUN
SI
GI BRGDS BELGIUM SLOT COORDINATION
    •  Separate lines for departures and arrivals.

 

SIR (Slot Information Request) - Coordinator's Response

The movements scheduled in the coordinator's list for the period from 1 to 7 June are listed in the following response.:

SIR
/ops@avionops.com
S25
27MAY
BRU
REYT/
HKF124 KF123 01JUN07JUN 0240060 262332 DXB0700 1000DXB CC
HAV127 05JUN 262332 YUL1000 J
H  AV126 06JUN 262332 0500YUL J
GI BRGDS BELGIUM SLOT COORDINATION
    •  Confirmed flights start with H (held).


Réferences

-WASG (Worldwide Airport Slot Guidelines) IATA defining global standards.

-The section of official IATA website dedicated to airport slots.

-Crash Course, a document from Chapter 6 of IATA's SSIM (Standard Schedules Information Manual) explaining SCR/SI messages.

 

 

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