Instrument Procedure Design Process

So far in this series of articles, the notion of instrument procedures – both approach and departures – has been presented as well as the navigation sources. The navigation sources include conventional (ground based) and satellite-based, both unaided satellite navigation (GPS) and augmented (WAAS). This article explains how a usable instrument procedure is created.

The design of instrument procedures in Canada is governed by Transport Canada. Until 1996 Transport Canada was the sole provider of this service. Transport Canada has not designed instrument procedures since that date and that service is provided by individuals and organizations that have received Transport Canada approved training.

Transport Canada&rsquot;s role is to create and regulate instrument procedure design criteria. This design criteria is, in essence, engineering instructions that describe the various parameters that have to be satisfied in order to meet the desired level of safety. The design criteria are found in a number of documents totalling thousands of pages. In other industries, the instrument procedure design criteria would be called design code or engineering orders.

The procedure design process starts with a data gathering phase. Topographic information, weather data, man-made obstacle location and heights and a host of aeronautic information is gathered before the design process begins. Fundamental and absolutely critical to the design of instrument procedures is knowing where the runway is located. While this might sound trivial, an accurate runway survey is critical as the entire design is anchored by the runway location. This is the case for satellite-based procedures and is particularly critical for augmented satellite procedures such as WAAS. At one time it was a matter of placing a handheld GPS device on the threshold and having the device take an average position over a period time. Today, the runway survey standards require either an Accredited Land Surveyor or Professional Engineer certify that the runway is surveyed to a high standard of accuracy and other parameters. This phase of the design process cannot be under emphasized as a poor runway survey will lead to an approach procedure that will have to be redesigned and possibly potentially flight checked a second time, both of which incur a financial cost.

Concurrently with the runway survey a number of other tasks should occur. A verification of the location and heights of any man-made obstacles in the area is a wise idea – frequently the official database of these obstacles is incomplete as towers are often erected without the authorities being notified. As well, obstacles near the runway need to be carefully measured to determine if the airport qualifies for two criteria – non-precision runway status and the Glide Slope Qualification Surface (GQS). Both of these issues will be explored in more detail in the eighth article in this series.

With this information, the designer applies the "design code" and develops an instrument procedure. Some organizations use traditional paper methods while others incorporate varying degrees of computer technology. This process can take a number of days or more, depending upon the complexity of the procedure and the environment. During this phase the designer would be coordinating their design with the air traffic control organization to insure that the contemplated procedure will integrate with the airspace in the region.

Once the design is completed, it has to be flight checked. The flight check serves a number of purposes for all types of instrument approaches. First, it verifies the fly-ability of the procedure design. Fly-ability includes such things as aircraft manoeuvring, descent gradients and runway alignment. If the procedure presents difficulties for fly-ability changes will be required. Sometimes procedures that meet the design criteria and look good on paper fail the fly-ability test in the real world. Communication with air traffic control is verified at the minimum planned altitudes; sometimes these minimum altitudes have to be raised to achieve radio reception. The flight check is also another opportunity to look for obstacles particularly in the critical phases of flight.

Flight checking also serves other purposes for particular instrument procedures. GPS (WAAS) are, for all intents and purposes, precision approaches. (Although WAAS (LPV) approaches are not technically not precision approaches, they use the same design criteria for the creation and from a pilot perspective they are flown identically as a precision approach.) The information that is coded into the aircraft database is subject to a Cyclical Redundancy Check (CRC). This is done to insure that it is decoded without any corruption of the critical information that describe the approach procedure. The flight check verifies that this occurs properly. The flight check also verifies that the WAAS satellites are visible and that they are not subject to any electromagnetic interference. WAAS flight checking, for these reasons, is a highly specialized activity requiring unique processes and data recording equipment.

ILS flight checking, also known as flight inspection, involves verifying that the quality of the electromagnetic signal from the ground based navigation aid meets certain performance specifications.

Following the flight check, the entire procedure is subject to a verification process by an independent party. The entire data package is then forwarded to NAV CANADA for processing. NAV CANADA has the sole legal authority to fulfill this role. At this stage, the depiction (approach chart) is verified then all of the appropriate data – depiction, database information and other aeronautic information – is forwarded to organizations worldwide for inclusion in paper products and database coding. It should be noted that neither NAV CANADA nor Transport Canada verifies the design of instrument procedures; the individuals or organizations with the Transport Canada approved training are able to do this internally.

Designing an instrument procedure requires knowledge of a wide ranging array of information that combines design criteria with obstacle information to create a mechanism to fly a departure or approach safely and effectively.