Approach Procedures — Page 165, Lesson 245

Let's get into the approach procedures chapter. We're starting with the big picture: how instrument procedures are classified. Broadly, ICAO splits instrument procedures into two types based on the guidance they provide. The first is a Precision Procedure, which is a runway approach, and it comes in Categories I, II, and III. The second is a Non-precision Procedure, which is an aerodrome approach. So the fundamental difference is precision versus non-precision. Let's unpack what "precision" actually means here. A precision approach procedure gives you two critical things. First, accurate track guidance, which we call azimuth — that's your horizontal steering, keeping you aligned with the runway centre line. Second, it gives you information about your height above the threshold of the runway, which is your elevation. In every case, you need external equipment to provide this data. You can't do it with just your eyes or basic instruments. Now, here's the key mechanism. By flying the required track and glide path within the required accuracy of half scale deflection of the course deviation indicator, or CDI, the aircraft stays within a protected area. That protected area is what guarantees terrain clearance, specifically above the OCA/H, which is the Obstacle Clearance Altitude or Height. So as long as you're within that half-scale deflection, you're safe from terrain. What equipment can provide this? ILS, which is Instrument Landing System, MLS, Microwave Landing System, and PAR, Precision Approach Radar, are all examples. In the USA, GLS, or GPS Landing Systems, are now in use, and early indications show accuracy in the order of plus or minus 30 centimetres to the centre line. That's remarkably precise. Here's a crucial design point: in the procedure design, obstacle clearance is implicit if you adhere to the descent path, the glide path. Because a precision approach terminates at the touchdown point, or at the commencement of a missed approach, it's often called a runway approach. Now, the pilot's responsibility. For a precision approach, you must calculate the height on final approach at which you make a decision: land, or go around and fly the missed approach procedure. This is called Decision Altitude/Height, abbreviated DA/H. It reflects the Operator's declared aerodrome operating minima, which is covered in Operational Procedures. Guidance on calculating DA/H is in the Operations Manual. Let me give you the exact definition of DA/H: it's the specific altitude, or height, in a precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. So the Operator must also specify the required visual criteria in the Operations Manual. Now, what happens if the glideslope fails? If the glideslope becomes unserviceable and you lose glide path indications, the approach automatically becomes a non-precision approach. That's a critical downgrade — you lose that vertical guidance. Finally, let's talk about precision categories. I want to emphasize this: at DA/H, if the approach has been flown correctly, the aircraft is exactly where it should be, so it must be safe. Further descent along track must also be safe. Think about it — if visibility were perfect, the aeroplane would be at the same place and height, and you'd continue the approach anyway. The system doesn't change the physics. Except where the system — meaning ground equipment and aeroplane equipment together — permits a "blind" landing, the latter stage of the final approach will be flown visually. The pilot needs visual reference to complete the landing. To accomplish this, a minimum RVR, or Runway Visual Range, is required, and also a visual reference. That's the threshold you need to see before you can continue below DA/H. So to tie it together: precision approach gives you both lateral and vertical guidance, you fly to DA/H, and if you don't have the visual reference and the required RVR, you go around. If the glideslope fails, you're now on a non-precision approach. That's the foundation of approach procedures.