Right, let's pick this up — Page 12, Lesson 12

Right, let's pick this up. We're now into the tolerances and the use of fleet values — this is the regulatory machinery that decides whether an aeroplane can actually stay in a fleet or has to be pulled out and treated on its own. Let me set the scene first. We've established a dry operating fleet mass and a fleet CG position — those are the reference values for the whole fleet. Now the rule says: if you weigh an aeroplane, or if you calculate the dry operating mass of an aeroplane in the fleet, and it varies by more than ±0.5% of the maximum structural landing mass from the established dry operating fleet mass — or if the CG position varies by more than ±0.5% of the mean aerodynamic chord from the fleet CG — then that aeroplane must be omitted from that fleet. Let me unpack those two tolerances carefully, because they're different quantities. The mass tolerance is ±0.5% of the maximum structural landing mass. That's a percentage of a specific structural limit, not a percentage of the fleet mass itself. So the allowable spread is tied to the aeroplane's structural landing limit. The CG tolerance is ±0.5% of the mean aerodynamic chord — the MAC — which is the reference chord length of the wing. So the CG position of each aeroplane must sit within half a percent of the MAC from the fleet CG position. Now, the rule doesn't just leave you stranded. It says separate fleets may be established, each with differing fleet mean masses. So if your aeroplanes naturally split into two groups, you can form two fleets, each with its own mean values. Then we get a refinement in (B). Suppose an aeroplane's mass is within the dry operating fleet mass tolerance — good — but its CG position falls outside the permitted fleet tolerance. The aeroplane may still be operated under the applicable dry operating fleet mass, but it must use an individual CG position. So you keep the fleet mass value, but you override the CG with that aeroplane's own figure. Now (C) — this is the case of a deliberate, known difference. If an individual aeroplane has, when compared with other aeroplanes of the fleet, a physical, accurately accountable difference — for example a galley or seat configuration — and that difference causes an exceedance of the fleet tolerances, then this aeroplane may be maintained in the fleet, provided that appropriate corrections are applied to the mass and/or CG position for that aeroplane. So the key words there are "accurately accountable" — you know exactly what the difference is, and you correct for it. Then (D) — a special case. Aeroplanes for which no mean aerodynamic chord has been published must be operated with their individual mass and CG position values, or must be subjected to a special study and approval. So if there's no published MAC, you can't use the fleet CG tolerance at all — you're back to individual values unless you get a special study approved. Now let's move to the use of fleet values — subparagraph (iii). After the weighing of an aeroplane, or if any change occurs in the aeroplane equipment or configuration, the operator must verify that this aeroplane falls within the tolerances specified in subparagraph 2(ii) — the ones we just covered. So every weighing, and every equipment or configuration change, triggers a re-verification against those tolerances. Then (B) — this is the ongoing management rule. Aeroplanes which have not been weighed since the last fleet mass evaluation can still be kept in a fleet operated with fleet values, provided that the individual values are revised by computation and stay within the tolerances defined in subparagraph 2(ii). So you don't have to weigh every time — you revise the individual values by computation, and as long as they stay inside the tolerances, the aeroplane stays in the fleet on fleet values. But here's the failure condition. If these individual values no longer fall within the permitted tolerances, the operator must either determine new fleet values fulfilling the conditions of subparagraphs 2(i) and 2(ii) — that is, re-establish the fleet basis — or operate the aeroplanes not falling within the limits with their individual values. So it's a binary choice: re-derive the fleet, or push the outliers onto individual values. Let me just make sure the logic chain is clear. The whole system rests on those two tolerances — ±0.5% of maximum structural landing mass for mass, and ±0.5% of mean aerodynamic chord for CG. Any aeroplane that breaches either one is omitted from the fleet, unless it's the accurately accountable difference case where you correct for it, or the mass-only breach where you keep the fleet mass but use an individual CG. And the ongoing rule is: verify after every weighing or configuration change, revise by computation in between, and if computed values drift out of tolerance, either re-establish the fleet or go individual. That's the complete regulatory picture for fleet tolerances and fleet value usage.