Principle 4. Demand and Pressure

Principle 4. Demand and Pressure


Demands and pressures relating to efficiency and capacity have a fundamental effect on performance

Performance needs to be understood in terms of demand on the system and the resulting pressures

"Systems respond to demand, so understanding demand is fundamental to understanding how the system works." Image: autowitch CC BY-NC-SA 2.0

Systems respond to demand, so understanding demand is fundamental to understanding how the system works. ATM demands are highly variable by nature, in type and quantity. Different units vary in their traffic demand, and traffic demand in the same unit varies enormously over the course of a day and year. Demand can come from customers (outside or inside the organisation) such as airlines and airports, or from infrastructure or equipment that provides a service.

A controller in a busy unit must meet demands from many pilots flying different types of aircraft, on various routes, using several procedures, in the context of dense traffic, to a tight schedule with little margin for disturbances. The controller must also meet demands from colleagues and technical systems. An engineer in the same unit may need to deal with various hardware and software with different maintenance schedules, as well as occasional unpredictable failures. All of this occurs under time pressure with variable resources.

Seddon (2005)[1] outlines two types of demand. The first is ‘value demand’. This is the work that the organisation wants; it is related to the purpose of the organisation and meets customer needs. Examples include a ‘right first time’ equipment fix, or training at the right level of demand to prepare staff for the summer peak in traffic. The second type is ‘failure demand’. This is work that the organisation doesn’t want, triggered when something has not been done or not done right previously. Often, failure demand can be seen where there is a problem with resources (e.g. inadequate staff, a lack of materials or faulty information). A temporary maintenance fix due to a missing spare part or lack of time will require rework. Training provided too soon in advance of a major system change may require repetition.

To understand system performance, it is necessary to obtain data about both demand and flow. Together, these measures will tell you about the system’s capability – its performance in responding to demand and the predictability of this performance. Some demand will be routine and predictable (in the short or long term) and there will often be good data already available (e.g. morning peak in traffic, the routine maintenance schedule). Other demand is less predictable (e.g. such as that associated with an intermittent fault on a network).

To respond to varying demand, people adjust and adapt. But, depending on resources, constraints, and the design of work, demand leads to pressure (e.g. from pilots, colleagues, supervisors, technical systems), and trade-offs are necessary, especially to be more efficient. Long-term or abstract goals tend to be sacrificed with increasing pressure to achieve short-term and seemingly concrete goals (such as delay targets).

For unusual events, it is important to get an understanding of demand (amount and variety) – both for the specific situation and historically. Understanding historical demand will give an indication of its predictability. But demand and pressure can only be analysed and understood with the people who do the work – the field experts. They can help you to get behind the numbers.

Designing for demand is a powerful system lever. To optimise the way the system works, the system must absorb and cater for variety, not stifle it in ways that do not help the customer (by targets, bureaucracy, excessive procedurisation, etc). It may be possible to reduce failure demand (which is often under the organisation’s control), optimise resources (competency, equipment, procedures, staffing levels), and/or improve flow. All meet customer needs, including of course the need for safety, and so address the purpose of the system.

Practical advice

  • Understand demand over time. It is important to understand the types and frequency of demand over time, whether one is looking at ordinary routine work, or a particular event. Identify the various sources of demand and consider the stability and predictability of each. Consider how field experts understand the demands.
  • Separate value and failure demand. Where there is failure demand in a system, this should be addressed as a priority as it often involves rework and runs counter to the system’s purpose.
  • Look at how the system responds. When the system does not allow demand to be met properly, more pressure will result. Consider how the system adjusts and adapts to demand, and understand the trade-offs used to cope. Listen to field experts and look for signals that may indicate trouble.
  • Investigate resources and constraints. Investigate how resources and constraints help or hinder the ability to meet demand.

View from the field

Massimo Garbini Chief Executive Officer, ENAV, Italy

“ENAV manages more than 1.8 million flights per year, with peaks of 6,000 flights per day. The demand on the ATM system is not to be under-estimated. With four area control centres, 40 control towers, 62 primary and secondary radars, and hundreds of navaids, it is a complex and demanding operation. But ENAV can count on about 3,300 employees, two thirds of which are in charge of operational activities. They enable us to cope with a variety of ever-changing demands – 24/7, 365 days a year. Demand is where everything starts, and so it needs to be understood carefully. But demand cannot be understood only from statistics. The field experts are the ones that understand demand and related pressures from a work perspective. So it is necessary to work together on the system in order to meet demand and achieve the best possible performance.”


  1. ^ Seddon, J. (2005). Freedom from command and control (Second edition). Vanguard.

Source: Systems Thinking for Safety: Ten Principles. A White Paper. Moving towards Safety-II, EUROCONTROL, 2014.

The following Systems Thinking Learning Cards: Moving towards Safety-II can be used in workshops, to discuss the principles and interactions between them for specific systems, situations or cases.

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