Before an aviation maintenance technician can accurately rig flight controls, weigh an aircraft, check dihedral, or perform a host of structural inspections, the aircraft must first be placed in a known, repeatable orientation called level flight attitude. This is not simply a matter of parking on flat pavement and calling it good — it is a precise procedure governed by the aircraft's manufacturer and enforced by FAA-accepted maintenance data. Every measurement taken with the aircraft out of level is potentially wrong, and a wrong measurement can lead to flight control misrigging, incorrect weight-and-balance data, or misalignment of critical structural members. Understanding how and why leveling is done is essential knowledge for the AMT General written examination and for safe maintenance practice.
This article covers the purpose of leveling, the types of leveling references manufacturers specify, the tools involved, the step-by-step procedure, and the common errors that trap both students and working technicians.
Why Leveling Is Required
An aircraft in flight assumes a specific geometric attitude that the manufacturer calls the level flight attitude or rigging position. The entire airframe — wing incidence, horizontal stabilizer incidence, control surface travel, landing gear geometry — is designed and measured relative to this attitude. When the aircraft sits on the ground, landing gear compression, tire inflation differences, ramp slope, and fuel load can all conspire to tilt the airframe away from that reference. To correct for all of these variables and restore the manufactured reference, the technician levels the aircraft before taking any measurement that depends on a known geometric baseline.
The FAA's Aviation Maintenance Technician Handbook — General (FAA-H-8083-30) makes clear that leveling is a prerequisite for weight-and-balance procedures as well as for control-surface rigging checks. The aircraft's Type Certificate Data Sheet (TCDS) and the manufacturer's maintenance manual are the authoritative sources that specify exactly how a given aircraft is to be leveled — there is no universal method that applies to all aircraft.
Types of Leveling References
Manufacturers use several different methods to define the level reference, and the technician must consult the aircraft's specific maintenance manual to identify which method applies.
Leveling Plates or Pads
Many aircraft have small machined or stamped leveling plates (sometimes called leveling lugs or pads) permanently attached to the airframe — often on the fuselage sidewall, door sill, or a structural bulkhead. These flat surfaces are machined parallel to the aircraft's longitudinal or lateral datum and are designed to accept a spirit level directly. When the bubble in a spirit level placed on the pad is centered, that axis is level.
Leveling Screws or Sighting Points
Some aircraft specify two or more leveling screws — threaded fasteners installed in the airframe at precise locations. The technician stretches a string or places a straightedge between the screwheads and measures with a level. Others may specify optical or laser sighting from defined reference holes or marks.
Plumb Bob Reference
On larger or older aircraft, a plumb bob is suspended from a defined overhead point, and the bob is compared to a reference mark on the floor or lower structure. When the string aligns with the mark, the aircraft is level in the specified axis. This method is particularly useful for checking lateral level (wing-to-wing) and for verifying symmetry during major structural repairs.
Electronic Inclinometers and Digital Levels
Modern shops frequently use digital inclinometers or electronic angle gauges. When placed on a certified leveling surface of the aircraft, these tools can resolve angles to within a fraction of a degree and provide repeatable, calibrated readings. However, the tool is only as accurate as the surface it is placed on — the manufacturer's specified leveling point must still be used.
Equipment Used for Leveling
The basic equipment list for leveling an aircraft typically includes:
- Spirit (bubble) level: A traditional glass-tube level used on leveling plates or sills. It must be recently calibrated and in good condition; a cracked or oil-contaminated tube gives false readings.
- Plumb bob and string: Used where vertical reference is required, and for checking longitudinal or lateral symmetry.
- Digital inclinometer: Preferred for accuracy and ease of use in modern shops; must be zeroed on a known flat surface before use.
- Adjustable jacks or support stands: Used to raise, lower, or tilt the aircraft. Most aircraft are leveled on jacks rather than on their landing gear, because jack height can be adjusted precisely. The aircraft's maintenance manual specifies approved jack points.
- Chocks and safety collars: Placed on jacks and around wheels to prevent inadvertent movement during the procedure.
The Leveling Procedure — Step by Step
While the exact steps vary by aircraft type, the general procedure follows a logical sequence that any AMT can internalize.
- Consult the maintenance manual. Identify the specific leveling method, leveling points, and any weight conditions required (for example, some manufacturers specify leveling with fuel tanks full, others with tanks empty, and still others with specific ballast installed).
- Prepare the work area. Position the aircraft in a hangar or on level, solid pavement away from wind. Wind can deflect a plumb bob string and rock a jacked aircraft dangerously.
- Install jacks at approved jack points. Jack the aircraft using only manufacturer-approved jack points to avoid structural damage. Raise the aircraft until the landing gear is just clear of the ground, or until the jacks are bearing full aircraft weight as specified.
- Place the level on the manufacturer's specified reference. Orient the level along the longitudinal axis (nose to tail) first to correct pitch attitude, then recheck along the lateral axis (wing tip to wing tip) to correct roll attitude. Some procedures require leveling both axes simultaneously.
- Adjust jack heights. Raise or lower individual jacks in small increments until the bubble is centered (or the digital readout shows zero) on both axes. Make adjustments slowly — rapid changes can shift the aircraft's center of gravity and destabilize the jacks.
- Verify and record. Once level is confirmed on both axes, re-verify by rechecking the longitudinal level after the lateral adjustment (one adjustment can affect the other on some aircraft configurations). Record the final jack heights and any relevant conditions in the maintenance record.
Special Considerations
Fuel and Fluid State
Fuel is heavy — aviation gasoline weighs approximately 6 pounds per gallon and jet fuel approximately 6.7 pounds per gallon. An asymmetric fuel load can shift the aircraft's lateral attitude enough to affect leveling accuracy. Always follow the maintenance manual's specified fuel state before leveling, and do not add or remove fuel once the aircraft is level and measurements have begun.
Hangar Floor Variations
Hangar floors are rarely perfectly flat. The jack-leveling procedure corrects for floor slope, but the technician must ensure the jacks are on solid footing and that no jack is perched on a drain, expansion joint, or soft spot. A jack that sinks or tilts can suddenly un-level the aircraft or, in a worst case, cause the aircraft to fall.
Temperature Effects
Tire pressure and strut extension change with temperature. If the aircraft will be leveled on its gear rather than on jacks (some light aircraft procedures allow this), the tires must be inflated to the specified pressure and the struts serviced to the specified extension before leveling begins.
Key Numbers and Rules
- The authority for leveling method and reference points is always the aircraft's specific maintenance manual and/or the TCDS — not a generic rule.
- Aviation gasoline weighs approximately 6 lb/gal; jet-A approximately 6.7 lb/gal — relevant to fuel-state requirements.
- Weight-and-balance calculations performed on an un-leveled aircraft are invalid and must be repeated.
- Jack points are airframe-specific; using unapproved points can cause structural damage.
- Both longitudinal (pitch) and lateral (roll) axes must be leveled, not just one.
- After adjusting one axis, always recheck the other axis — adjustments interact.
Common Test Traps
- Assuming all aircraft use the same leveling method. The FAA exam tests whether you know that leveling method is aircraft-specific, defined by the manufacturer's maintenance manual. Never assume a method applies universally.
- Forgetting to check both axes. Some students focus only on longitudinal level because the aircraft has nose-to-tail length as a reference, and overlook the lateral axis. Both must be confirmed.
- Confusing the level reference surface with any convenient flat surface. Placing a level on a skin panel, access door, or cargo floor that is not a manufacturer-specified leveling point will give an incorrect reading. Only approved leveling pads, lugs, or specified structural points are valid.
- Neglecting fuel state. A common distractor question describes a scenario where the technician levels the aircraft and then fuels it asymmetrically. The correct answer is that leveling must be re-verified or performed with fuel in the specified state.
- Not re-verifying after each adjustment. Adjusting lateral level can shift longitudinal level slightly on some aircraft. Always make a final check of both axes after all adjustments are complete to confirm the aircraft remains in the correct rigging attitude.