Aerodrome Lighting — Page 426, Lesson 577

Let's pick up with the PAPI system — the Precision Approach Path Indicator. We've covered the light units themselves, so now I want to walk you through the indications, the special case of circling approaches, and the critical concept of Minimum Eye Height, or MEHT. First, the indications. The PAPI system uses four light sets. The possible combinations of those four light sets give you five different indications, and each one tells you where the aircraft — more precisely, where the pilot's eye — sits with respect to the defined glide path. So you have: well above, slightly above, on or close to the glide path, slightly below, and well below. That's the full set of five. The key point here is that the reference is the pilot's eye, not the aircraft's wheels or the fuselage — that distinction becomes very important in a moment. Now, circling approaches. When a runway is used for circling approaches — that's visual manoeuvring circling, where you fly a visual pattern around the field — a PAPI set is placed on either side of the runway. Normally, there would only be one set, on the left-hand side. The reason for this placement is that it allows the pilot to use the PAPI wing bars as a horizontal reference for aircraft attitude during the later part of the circling manoeuvre. So the wing bars aren't just for glide path guidance here — they give you a visual horizon reference to keep the wings level as you complete the circling turn. Now let's get to the really important part — Minimum Eye Height, or MEHT. Here's the problem. If the PAPI system were located exactly at the threshold of the runway, and the pilot flew a visual approach keeping the aircraft exactly at the 'on glide path' position, the wheels would hit the ground before the aircraft reached the threshold. Why? Because the pilot's eye is not at the bottom of the aircraft — the undercarriage is below and behind the eye. So the distance before the threshold where the wheels hit the ground would be a function of the distance from the pilot's eye to the bottom of the undercarriage. That's the 'eye to undercarriage' distance. To overcome this, the visual aiming point — which is coincident with the PAPI location — is set a distance down the runway. Here's the specific figure: on code 4 instrument runways at least 2400 metres in length, the aiming point is set 400 metres from the threshold. In that case, for a normal 3° glide path, the pilot's eye would be 65 feet above the surface on crossing the threshold. So that 65 feet is the clearance you get at the threshold, given the 400-metre setback and the 3° path. Now, there's a subtlety here. This calculation assumes that the 'on glide path' indication is the 'mid angle' — the exact middle of the range. But in reality, the 'on glide path' indication is not a single angle; it's encompassed in a bandwidth of angles within which that indication is visible. So the pilot could be at the lowest possible 'on glide path' indication and still be showing 'on glide path'. Here's why that matters. If you can establish the height of the pilot's eye when the aircraft is over the threshold with the lowest possible 'on glide path' indication, that gives you a way to determine whether an aircraft can use the PAPI system as set up for that runway. That lowest possible height is the figure quoted on the ICAO aerodrome chart — that's the MEHT, the minimum eye height — and it's printed alongside the location of the PAPI on the chart. So the MEHT is the minimum height at which the pilot's eye can be, over the threshold, while still showing the 'on glide path' indication. If your aircraft's eye-to-undercarriage distance means your wheels would strike before the threshold even at that minimum eye height, then the PAPIs aren't usable for your type on that runway. And that's where type rating instruction comes in. During type rating, you'll be made aware of the 'eye to undercarriage' requirement of your specific type, so that you can assess the usability of the PAPIs for the aircraft you're flying. That's the practical application — you take the MEHT from the chart, you know your eye-to-undercarriage geometry, and you decide whether the system works for you. So to tie it together: five indications from four light sets, the pilot's eye as the reference, the left-side placement for circling approaches, and the MEHT as the safety figure that accounts for the eye being above the wheels. That's the complete picture of how PAPI indications and their limitations work.