In operation, five fundamental load types act on an aircraft's structure:
| Load | Effect | Aircraft example |
|---|---|---|
| Tension | Pulling apart along an axis | Bracing cables, mount points |
| Compression | Pushing together along an axis | Landing-gear struts at touchdown, spar upper surface in positive g |
| Shear | Sliding in opposite directions | Rivets, bolts, glue joints |
| Torsion | Twisting about an axis | Engine torque on the mount, wing under asymmetric load |
| Bending | Combination of tension + compression on opposite sides | Wing under lift load (upper compression, lower tension) |
In practice several loads usually act simultaneously — designers size parts for the worst combination. Materials behave differently: aluminium alloys are good in tension and compression; pure compression members (spar webs) are often thick aluminium or composite.
Load factor n
The load factor n describes the relationship between lift and weight (flight mass) of the aircraft:
n = Lift / Weight (or L / m·g)
- Level flight: lift = weight → n = 1 ("1 g").
- Steep turn 60° bank: lift = 2 × weight → n = 2 ("2 g").
- Top of a loop: pilot feels weightless, lift < weight → n < 1 or negative.
- Vne pull-up: lift far greater than weight → n up to the structural limit.
The load factor is the measure of structural stress on the aircraft.
Structural limits — limit load and ultimate load
For CS-23 certification two load limits are defined:
| Term | Meaning |
|---|---|
| Limit Load | The maximum load that may occur in normal flight operation. At this load no permanent deformation. For CS-23 Normal Category: typically +3.8 g / -1.52 g; Utility +4.4 g / -1.76 g; Aerobatic +6 g / -3 g. |
| Ultimate Load | The design value at which the part may fail in a fracture test — defined as 1.5 × Limit Load (safety factor 1.5). At this load the structure may fail. |
Example (CS-23 Normal):
- Limit Load = +3.8 g
- Ultimate Load = 3.8 × 1.5 = +5.7 g
→ The aircraft must, in a test, be loaded to Ultimate Load without immediate failure (held 3 seconds). Beyond that, failure is permitted.
Manoeuvring speed (Va) and gusts
At speeds above the manoeuvring speed (Va) the wing can no longer protect against structural limit load by stalling first. Consequence:
- At or below Va: at full elevator deflection the wing stalls before limit load is reached → structurally safe.
- Above Va in heavy gusts: a sudden vertical gust can damage the fuselage structure (especially wing and tail) via the resulting load factor.
→ In expected turbulence, reduce speed to Va or below — a standard measure to avoid structural damage.
Minimum equipment — defined in the AFM
The minimum equipment of an aircraft is the equipment specified in the flight manual (AFM / POH) that must be present and operational for the relevant operation (VFR day, VFR night, IFR). Without this equipment the flight is not allowed.
Consequence: the pilot checks before every flight that all AFM-required items work — on defects the Minimum Equipment List (MEL) is consulted (for multi-engine / complex aircraft) or the flight is cancelled.
Primary Flight Display (PFD) — aircraft guidance
A Primary Flight Display (PFD) is an electronic cockpit display in modern glass-cockpit avionics (e.g. Garmin G1000, Avidyne Entegra). It shows information about aircraft guidance in integrated form:
- Attitude indicator (pitch/bank).
- Airspeed tape (IAS with Vne, Vno, Vfe, Vs markings).
- Altimeter (altitude with QNH subscale).
- Heading indicator (magnetic heading).
- Vertical speed indicator (climb/descent rate).
- Slip/skid indicator (coordination).
- HSI with nav data (CDI, course, bearing).
→ The PFD replaces the "six pack" of standard instruments with an integrated digital presentation.