How do pilots find an airport in poor weather conditions or at night? How can airplanes safely depart an airport and fly into weather or into the nighttime and not hit any obstacles? This is all achieved through the wonders of instrument flight.
Unlike the early barnstorming days when pilots flew by the seat–of–the–pants, instrument flight occurs the pilots must rely upon the flight instruments in the cockpit to provide vital information such as attitude, airspeed, direction and a host of other vital pieces of information. Without the instruments providing this information, flight in adverse weather or nighttime would be impossible.
While the aircraft instruments help the pilot in flying the aircraft, assistance is also required in navigating the aircraft. In the next couple of articles the menu of navigation aids will be discussed but in this article we will describe how these navigation aids are used to successfully fly from one airport to the next.
An instrument departure is a procedure that takes an aircraft from lift–off until it achieves a safe cruising altitude. Instrument departures are as important as instrument approaches – aircraft departing from an airport encounter risks just as aircraft arriving at airports do. The risks for a departing aircraft are natural terrain or man–made obstacles that an aircraft could collide with if the aircraft either does not have sufficient climb capability to clear them.
There are a couple types of departure used for aircraft in instrument flight. The simplest one is a diverse departure – this type of departure assumes that an aircraft climbs straight ahead after lift–off to 400 ft above the airport before any turns are attempted. And while the aircraft is climbing it is assumed that it climbs at a rate of 200 ft vertically for every nautical it travels over the ground. Often, terrain and/or towers penetrate these protected areas and in this case, special departures are created to provide guidance to the pilot to avoid these obstacles. In some cases, navigation guidance from a ground-based navigation is used and, more recently, navigation guidance from the Global Positioning System (GPS) is used to define a path over the ground that would avoid any obstacles that would preclude a safe departure and climb to cruise altitude.
At the other end of the flight, a mechanism must be available to allow the pilot to find the runway and acquire it visually to accomplish a safe landing. These are instrument approach procedures.
Instrument approaches come in a wide variety of flavours. Until recently, they have all been associated with a navigation aid that was installed on the ground – a terrestrial navigation aid. These will be discussed in more detail in the next article. Recently, however, satellite–based navigation using GPS have been introduced. This navigation source will be discussed extensively in the proceding pages.
Regardless of the navigation aid used – terrestrial or satellite – an Instrument Approach Procedure (IAP) must be designed in concert with it. An IAP is an engineered path for an aircraft to take it from the enroute phase of flight to the runway or perhaps the airport environment. This path has a variety of protected areas both vertically and laterally that guarantee that an aircraft will not come in contact with any terrain or other obstacles. In many ways it is like a road designed by an engineer – it has to be safe, consistent with a design code and tested before it is used.
How low an aircraft can descend or how close it can get to the runway is known as approach minimums. The value that can be achieved for a particular runway depends on the navigation aid used for the procedure design (each of them have different qualities in terms of navigation accuracy, physical location of the device), the local terrain and obstacles, the runway status, the presence of a local altimeter and so on.
Approach minimums are expressed two ways. Generally, approach minima is the altitude (in feet above sea level) that an aircraft can descend to. When the aerodrome or runway elevation is subtracted from the value, the approach minima above the airport is then determined. This value (in feet) is correlated with cloud ceiling – this is what determines if an approach is possible with a reasonable expectation of success. If the cloud ceiling is lower than the minimum altitude that the instrument procedure permits, it is unlikely that a successful approach is possible.
Minima are also expressed in terms of visibility (in statute miles) that must be available for the pilot to acquire the landing environment.There are two types of instrument approaches – precision and non–precision. Precision approaches provide the aircraft with an extremely precise navigation signal that provides both a lateral and vertical path to the runway. Examples of precision approaches are the Instrument Landing System (ILS), Microwave Landing System (MLS) and more recently, the satellite-based Wide Area Augmentation System (WAAS).
Non–precision approaches use a navigation signal that does not have the same qualities of accuracy and do not provide vertical guidance to the pilot. All other instrument approaches fall into this category.
The important thing to remember is that a navigation aid alone does not provide an instrument approach to an airport. An instrument approach procedure, designed using the design instructions or criteria, is created in concert with a navigation aid. You cannot have one without the other.