Airport Issues and Instrument Procedures

Instrument procedures are the airborne portion of a flight whereas the runway environment provides a critical link to the ground borne portion. For an aircraft departing an airport, the runway environment from the moment the aircraft leaves the ground to when it is safely climbing away is equally important as the environment from when an approaching aircraft acquires the runway visually and performs a safe landing.

A clear departure area at the end of a runway is ideal. Often, this cannot occur due to the presence of natural or man-made obstacles. Sometimes the cost of removing these obstacles is prohibitive; other times these obstacles are outside the legal control of the airport and in other cases it is virtually impossible to remove the obstacles. In most cases, these situations can be accommodated using a variety of techniques. This may include a visual climb over the airport before departing the airport area, a note to the pilots to be aware of certain obstacles, etc.

For approaching aircraft, the runway environment is important for many reasons. The time period from when an aircraft completes an instrument approach to landing is matter of seconds. Little time or limit ability is available to dodge obstacles just prior to landing.

Another important issue the airport status as this limits the instrument approach minimums for an instrument approach procedure.

Airports and runway environments have three general classifications. A Non-Instrument airport is the most basic airport classification – this is the default class of airport. Airports with this classification are permitted to have instrument approach procedures to as low as 500 ft above the airport. It should be noted that the instrument approach minimums are the highest of the instrument design or the airport classification.

The next higher level of airport status is a Non-Precision classification. Airports that meet this criteria can allow approach procedures to as low as 250 ft above the airport. Finally, the Precision classification is for airports that want to support instrument approaches to Category I ILS (200 ft minimums) or lower. It should be noted and emphasized that WAAS/LPV approaches, despite their strong similarities to ILS, are for the purposes of airport/runway classification considered to be non-precision approaches.

The airport standards to determine the classification is determined the type of airport. Certified Airports are governed by TP 312; Registered Aerodromes are governed by an Aerodrome Attestation. TP 312 uses the runway length to determine the parameters that must be satisfied to achieve a particular status. The Aerodrome Attestation uses the aircraft wingspan to determine the parameters that must be satisfied to achieve a particular status. This is subtle but important difference between Certified Airports and Registered Aerodromes.

In order to advance from a Non-instrument runway to a Non-precision runway, certain obstacle free areas must be satisfied. The concepts are identical for certified airports/registered aerodromes. Transitional Surfaces – those perpendicular to the runway centerline – are identical in application between the two types of airports/aerodromes however the exact values may vary. Similarly, the Obstacle Limitation Surface (OLS) – the area in the approach and departure areas of a runway – are identical conceptually but with different values.

Advancing a runway from a Non-precision to Precision status requires a huge increment in complexity and cost. This includes obstacle clearance areas but also approach lighting. The only benefit from an approach perspective is perhaps a 50 feet reduction in approach minimums from 250 ft to 200 ft – a very expensive 50 ft! It should also be noted that WAAS/LPV approaches do not have any special lighting requirements.

All instrument approach types with vertical guidance (ILS, Baro VNAV, LPV, RNP) require that a narrow area from approximately 4000 ft prior to the threshold to the threshold itself meet another, slightly more restrictive obstacle clearance area than the OLS. This area is known as the Glideslope Qualification Surface (GQS). This protected area is intended to provide pilots comfort that once the aircraft acquires the runway during a vertically guided approach that no obstacle will interfere with the continuous descent to the runway threshold.

What is the process to assess the airport to satisfy the TP 312/Aerodrome Attestation and GQS requirements? A site visit by a trained and experienced aerodromes expert or procedure designer with the appropriate survey and laser ranging equipment is required. This type of work cannot be accomplished remotely. During this process, all of the obstacles are in and around the runway are measured for their distance/elevation and are assessed to determine whether they satisfy the requirements.

An airport/aerodrome with a non-precision runway status satisfies the ground portion of an instrument procedure design making the airport eligible for instrument approaches to as low as 250 ft above the airport. An airport with a satisfactory GQS will allow vertically guided approaches to be developed. A non-precision runway in combination in most cases will provide an airport accessibility of over 98%. That is, the period of time when the instrument approach minimums are lower than the reported weather. And with satellite-based navigation, there is no ground infrastructure cost, just the cost of the instrument procedure design. For the vast majority of airports a non-precision runway status provides the optimum result. But even airports that only get a non-instrument classification, instrument approach minimums to as low as 500 ft are still a big improvement over what the existing approach may offer, if there is any instrument approach at all.