Aircraft General and Ground Geometry

ATA-20 is not a system chapter. Every other chapter describes hydraulics, electrics, fire protection — a set of components that do something. ATA-20 describes the aeroplane itself: what kind of aircraft it is, how big it is, which parts of the fuselage are not pressurised, where the ground service points sit, how tightly it turns on the ground, and how high the pilot's eye is above the wheels at touchdown. This looks like background, but it is the mental picture a crew draws on for every walk-around, every gate entry, every runway turn-round, and every low-visibility landing.

FCOM files this as DSC-20 Aircraft General, in six sub-chapters that run from "what it is" to "how big" to "how it moves":

                    ATA-20 AIRCRAFT GENERAL — the aeroplane itself
   ┌────────────────┬────────────────┬──────────────────────────────┐
   what it is        how big / where        how it moves + how you see it
   20-10 Overview    20-20 Description      20-30/40  +  20-50/60
        │                  │                      │              │
   subsonic,          principal dims (fig)   turning radii    ground        landing    visual
   medium-long        unpressurised (fig)    180° turn width  clearance     geometry   ground geom
   2 turbofans        antenna (fig)          NWS 72°          (tailstrike/  (eye/GS/   (cut-off
   under wings        17 service points                       nacelle)      main gear) over nose)
   2+2 cockpit             │                      │                 └──────┬───────┘
   cabin variable          │                      │              widebody "sits high" + CAT III RVR
   cargo 3/4         walk-around map        gate / turn-round planning     low-vis "how much runway"

The rest of this article walks the six sub-chapters. A companion article, Operating Limitations, covers the other half of "Aircraft General" — the limits (speeds, weights, load factor, wind).

[!warning]- Most of DSC-20 is drawings, not text The principal dimensions, ground-clearance, landing-geometry and visual-geometry sub-chapters are almost entirely engineering figures — the numbers live on the drawings, not in prose. This article quotes only what FCOM states in text and the operating numbers printed on the figures (turning radii, 180° turn width, CAT III RVR). Specific spans/lengths/heights are not reproduced here — read the three-view for the model you fly.

1. What the aircraft is (DSC-20-10 Overview)

FCOM paints the aircraft in five sentences:

"The A330 is a subsonic, medium to long range, civil transport aircraft."

"The aircraft has two high bypass turbofan engines, mounted under the wings."

"The cockpit is arranged for a two-member crew, and also has a place for two observers."

"The layout for cabin occupants seating may be varied to suit operating requirements."

Cargo is where the first configuration split appears. On the passenger aircraft:

"Three cargo compartments are under the cabin floor."

On the freighter there are instead "There are four cargo compartments:" — "One main deck cargo compartment" plus "Three lower deck cargo compartments." The extra main-deck compartment is the whole passenger level converted to freight; it is also the stage for the freighter's depressurisation firefighting.

Learning hook. Recognise the aircraft in one breath: subsonic medium-long-range / two under-wing turbofans / 2+2 cockpit / variable cabin / three cargo holds (pax) or four (freighter). The 3↔4 cargo split is the first anchor for configuration awareness that runs through the whole knowledge base.

2. Description — dimensions, unpressurised zones, antennas, service points (DSC-20-20)

FCOM states the scope of this sub-chapter in one sentence:

"This subchapter gives the principal dimensions of the aircraft, the location of unpressurized areas, antennas, ground service connections and the ground maneuvering characteristics."

Principal dimensions. Span, fuselage length, height and track are on the three-view — figures, not text, in every FCOM. The -200 and -300 differ mainly in fuselage length (the -300 is longer, carries more, and therefore turns wider — see §3). Specific metres are not reproduced here; read the three-view for the applicable model.

Unpressurised compartments. The fuselage is not pressurised end to end: a sealed "tube" is the pressure vessel, and the structure outside it is unpressurised. A crew needs to know where that boundary lies because those areas have no pressurisation, no temperature control and no oxygen, and because decompression logic and consequences all hang on the pressurised-vs-unpressurised boundary. FCOM gives the exact outline as a figure; the specific parts are not listed here. The pressurisation system itself is ATA-21.

Antenna locations. Communication, navigation and surveillance antennas are distributed along the fuselage; FCOM gives their positions as a figure, cross-referenced to the RF systems in navigation (ATA-34) and communications (ATA-23).

Ground service connections and panels. This is the one part of the sub-chapter that is a written list, and the most useful for a walk-around. FCOM numbers the points (the list varies slightly by fuselage side and configuration; a representative set):

Learning hook. There are three hydraulic ground connectors in three colours — yellow, green, blue — one for each of the three independent hydraulic systems. Seeing three differently-coloured hydraulic points on the walk-around is the three systems surfacing on the fuselage. This service-point map is the geographic index for the walk-around: each point belongs to a system chapter, and ATA-20 is where they are named on the airframe.

3. Ground handling — turning radii and the 180° turn (DSC-20-30)

The daily question this answers: how tightly does this aircraft turn on the ground, and can it turn round on this runway?

Nose-wheel steering (NWS) limit angle is 72°. At that angle FCOM publishes minimum turning radii in two sets, one per fuselage length (the symbols Y and A, R3–R6 are measured points on the figure). The shorter -200 has the tighter set; the longer -300 the wider set:

"The above figure assume symmetric thrust and no differential braking."

That is the geometric minimum under standard handling; asymmetric thrust or differential braking can tighten it, but that is technique, not the baseline.

180° turn width is the headline operating number, and FCOM prints it against the model:

"For the A330-200, 41 m (133 ft) without margin" … "For the A330-300, 48 m (156 ft) without margin."

The conditions are the recommended 180° turn technique, a dry runway, and full 72° NWS. FCOM adds:

"The flight crew should consider additional margin when the runway is wet or contaminated."

The technique itself is in FCTM/PR-NP-SOP-100.

Learning hook. Remember the two numbers with their conditions: 41 m (-200) / 48 m (-300), dry, no margin, full 72° NWS. Check them against runway width before committing to a turn-round — many narrow runways need a turn pad, or do not qualify at all. "No margin + dry" is the usage note; wet or contaminated needs extra.

4. Ground clearance (DSC-20-40)

FCOM gives the height above ground of the various points (nacelle bottom, wingtip, tail, door sills) as a figure. AMM 06-10-00 adds two written caveats:

"Dimensions in the tables are approximate and will vary with tire type, W&B and others special conditions."

"Passenger and cargo door ground clearances are measured from the center of the door sill and from floor level."

The first is exactly why there is no single number to memorise; the second is the measurement datum. Two safety questions follow:

• Tailstrike — the tail-to-ground margin at rotation/flare drives the rotation rate and the landing attitude ceiling. A long fuselage makes the tail margin more sensitive than on a narrowbody.
• Nacelle / wingtip contact — the under-wing nacelles sit low, which governs the scrape risk in crosswind or bank, and the FOD/surface-damage sensitivity.

Specific clearances are on the figure and are approximate; they are not reproduced here.

Learning hook. The ground-clearance figure is the list of where this aircraft is closest to the ground. It explains why rotation must not be rushed, why a high-bank landing scrapes the tail, and why the low nacelles are FOD-prone. Stress the AMM line — clearance is not a constant, it varies with tyre and weight, so do not memorise it as a fixed number.

5. Landing geometry (DSC-20-50)

This sub-chapter turns the "widebody sight picture" into geometry, across two figures.

ILS final approach and landing geometry describes the geometric relationship between the pilot eye position, the glideslope (GS) antenna and the main landing gear. The intuition: a widebody sits high — the eye is well above the main wheels, and the GS antenna is offset fore/aft from them. On a given ILS glidepath the antenna rides the beam, so the threshold crossing height of the main wheels is set by this geometry, and the eye is higher again. This is why a widebody still "looks high" when the wheels are already near the threshold, and why the flare's visual height cue differs from a narrowbody. Specific eye heights and offsets are on the figure and are not reproduced here.

Minimum visual ground segment (flare phase) converts the visual requirement into RVR. FCOM gives the result, and two drawings correspond to two variants:

"for a CAT III landing (60 m minimum visual segment), the minimum RVR is 114 m at 15 ft."

"for a CAT III landing (60 m minimum visual segment) the minimum RVR is 104 m at 15 ft."

So a CAT III landing requires the pilot to see at least 60 m of ground segment in the flare; referred to a 15 ft eye height, that maps to a minimum RVR of 104–114 m depending on the variant. This is the purely geometric origin of the low-visibility RVR minimum — it is not arbitrary, it is derived from "from this eye height, needing to see this much runway lighting."

The operational reality, per the Airbus Getting to Grips with CAT II/III brochure:

"In Category III, pilots see the runway lights only few seconds (about 5 seconds) before touchdown, therefore there is no margin for error."

The 60 m segment / ~104–114 m RVR geometry, felt in time, is "those 5 seconds."

Learning hook. Crews often rote-learn the CAT III RVR minimum. Show them the drawing instead: sit at 15 ft, need to see 60 m of runway in the flare → need roughly 104–114 m of visibility. The RVR limit turns from a regulation number into your eye's geometry; add the "only ~5 seconds" line and the geometry gains its weight in time. Sitting high is double-edged: the view is good, but the eye-to-wheel offset means the flare judgement must be recalibrated.

6. Visual ground geometry (DSC-20-60)

FCOM gives, as a figure, the range of ground visible over the nose (a cut-off-angle picture): because the nose and glareshield obstruct the view straight down, the nearest visible ground point is some distance ahead of the nose — a blind zone. It pairs with §5: one covers "how far you see on approach/flare," the other "how near you see over the nose in normal operations." It matters for taxi (whether you can see near the nose wheel, follow the lead-in line) and close-in visual judgement. Specific angles/distances are on the figure.

Learning hook. There is a strip of ground ahead of the nose you can never see — which is why taxi relies on lead-in lines and marshalling, and why a close-in object (an obstacle, ground crew) sinks into the blind zone.

7. Why a "background" chapter matters to the crew

ATA-20 carries no procedure, but it is the geometric foundation of several daily operations:

1. Walk-around — the service-point map (§2) plus the ground clearances (§4) are what to look at, and where. The three coloured hydraulic points, the oxygen point and the fuel points are the geographic index of the walk-around route.
2. Gate / taxi / turn-round planning — the turning radii and 180° turn width (§3; 41 m for the -200, 48 m for the -300, dry, no margin) decide "does this taxiway take the turn?" and "can this runway be turned on?".
3. Rotation attitude and tailstrike awareness — the ground clearances (§4) explain the rotation rate and the landing-attitude ceiling; the landing geometry (§5) explains the widebody's high-seat visual bias.
4. Low-visibility operations — the landing geometry (§5) reduces the CAT III RVR minimum to "see 60 m of segment from a 15 ft eye," the key to understanding rather than memorising the limit.

Configuration awareness starts here: -200 vs -300 (fuselage length drives dimensions, turning radii, 180° turn width, landing RVR — always ask "which model?"), and passenger vs freighter (three vs four holds, main deck or not). Where the model matters, take the value for the aircraft you fly from its FCOM/AFM.

Self-test

[!note]- Q1. Paint the A330 in FCOM's five sentences — role, engines (type + position), cockpit crew, cabin layout, cargo. Why do passenger and freighter differ in cargo count? Subsonic medium-to-long-range civil transport; two high-bypass turbofans under the wings; two-member crew plus two observers; cabin seating variable to suit operations; three lower-deck holds (passenger) versus four (freighter: one main deck + three lower deck — the passenger level converted to freight).

[!note]- Q2. What is an "unpressurised compartment," and why must a crew know where it is? Structure outside the sealed pressure vessel. It matters because those zones have no pressurisation/temperature control/oxygen, and because decompression logic and consequences hang on the pressurised-vs-unpressurised boundary (tie to ATA-21).

[!note]- Q3. Why are there three hydraulic ground connectors, in three colours? Which chapters do the oxygen and fuel points belong to? Yellow, green, blue = the three independent hydraulic systems surfacing on the airframe. Oxygen point → ATA-35; fuel gravity/refuel points → ATA-28.

[!note]- Q4. NWS limit angle? 180° turn width for the -200 and -300? What two conditions attach to those numbers, and what changes when wet? 72°. 41 m (133 ft) for the -200, 48 m (156 ft) for the -300 — dry runway, no margin, full 72° NWS. Wet or contaminated requires additional margin.

[!note]- Q5. Why does a widebody "sit high"? What sets the main-wheel threshold crossing height? Where does the CAT III RVR minimum come from? Eye is well above the wheels, GS antenna offset fore/aft; on a given glidepath the antenna-on-beam geometry sets the main-wheel threshold crossing height. The CAT III RVR minimum (≈104–114 m at 15 ft) is derived geometrically from needing to see a 60 m ground segment in the flare — not an arbitrary number.

Key takeaways

References

• Per FCOM DSC-20-10 — overview: aircraft type, engines, cockpit, cabin, cargo (passenger three holds / freighter four).
• Per FCOM DSC-20-20 — description scope; principal dimensions, unpressurised areas, antennas (figures); ground service connections and panels (numbered list).
• Per FCOM DSC-20-30 — ground handling: minimum turning radii (symmetric thrust, no differential braking) and 180° turn width (-200 41 m / -300 48 m, dry, no margin); technique in FCTM/PR-NP-SOP-100.
• Per FCOM DSC-20-40/50/60 — ground clearance, landing geometry (ILS geometry; CAT III 60 m segment → RVR 104/114 m at 15 ft) and visual ground geometry (figures).
• Per AMM 06-10-00 — dimensions are approximate and vary with tyre type and W&B; door clearances measured from door-sill centre at floor level.
• Per Airbus Getting to Grips with CAT II/III §1.1 — in CAT III the runway lights are seen only about 5 seconds before touchdown.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.