Aircraft Control Cable Tension and Temperature Correction

Why an aircraft control cable's rig tension is specified against a temperature, and roughly what the correction is worth. The differential expansion is the whole reason for the manufacturer's rigging chart: an aluminium airframe expands roughly twice as much as a steel cable for the same temperature rise, so as the aircraft warms the structure stretches the cable system and tension CLIMBS. Rigging to the nominal tension on a hot ramp therefore leaves the system slack when it cools, and rigging to nominal on a cold morning leaves it over-tensioned in the sun -- which is why the chart gives a lower target at low temperature and a higher one at high. Both error directions are computed here, because they fail differently. Over-tension loads pulleys, bearings and fairleads continuously, raises control forces, and can exceed the system's design tension. Under-tension gives slack, which means lost motion at the control surface and, more seriously, reduced flutter margin -- so the two are not symmetric annoyances and neither is a small deviation. The instrument belongs in the same conversation as the arithmetic. A tensiometer reading is only correct if the riser and the calibration card match the cable's DIAMETER AND CONSTRUCTION: a 1/8 in 7x19 cable read on a card for 7x7 gives a wrong number with no indication that anything is amiss, and no amount of temperature correction rescues a reading taken on the wrong card. This is an ESTIMATE of the correction's magnitude from entered coefficients, cable area and an effective modulus, and it is not the chart. A stranded cable's effective modulus is well below solid steel's and varies with construction and with how thoroughly the cable has been pre-stretched, the structure's effective expansion depends on the load path rather than on the material alone, and the real system includes turnbuckles, quadrants and pulleys that this does not model. The aircraft maintenance manual's own rigging chart is the authority, the tensiometer's calibration card governs the reading, and a certificated mechanic performs and signs for the rigging.

the tension change is (alpha_structure - alpha_cable) x the temperature difference x the cable's metallic area x its effective modulus; the target at an ambient temperature is the nominal plus that change, and rigging to nominal instead leaves the system off by the change to the service temperature.

The differential thermal expansion that makes a rigging chart necessary -- an aluminium airframe expands roughly twice as much as a steel cable per degree, so tension RISES as the aircraft warms. Coefficients, cable area and effective modulus are ENTERED; a stranded cable's effective modulus is well below solid steel's and varies with construction and pre-stretch. An ESTIMATE of the correction's magnitude, NOT the chart.

One differential-expansion relation on entered properties.

Estimate. AHJ and licensed professional govern.

Field names used by the API: nominal_tension_lb, reference_temp_f, ambient_temp_f, cable_area_in2, cable_modulus_psi, structure_alpha_per_f, cable_alpha_per_f, service_temp_f, temp_difference_f, tension_change_lb, target_at_ambient_lb, tension_when_warm_lb

• An estimate, not the chart the maintenance manual's own rigging chart is the authoritythe aircraft maintenance manual
• Effective modulus is entered a stranded cable's is well below solid steel's and varies with pre-stretchthe cable manufacturer
• The reading needs the right card riser and calibration card must match diameter and constructionthe tensiometer's calibration card