Tyre Pressure versus Temperature
A main-wheel tyre (volume 0.055 m³) is serviced to the manual figure of 210 kPa gauge at a ramp temperature of 15 °C. The same manual states a maximum permissible inflation pressure of 250 kPa gauge, at which the wheel's thermal fuse plugs are designed to melt. On the previous turnaround this tyre was recorded at 262 kPa gauge after a long taxi in a 40 °C ambient and was signed off as 'within limits'. The aircraft is due to fly from the 15 °C base to an airport at −30 °C, where the manual requires at least 150 kPa gauge for bead seating. Take atmospheric pressure as 101.3 kPa and treat the tyre volume as constant.
1. Estimate the gauge pressure after a fast taxi raises the gas temperature to 60 °C. Has the fuse-plug limit been exceeded?
Read the worked answer
Constant volume, so use absolute pressures: P₂ = P₁T₂/T₁.
- P₁(abs) = 210 + 101.3 = 311.3 kPa
- T₁ = 288 K, T₂ = 333 K
- P₂(abs) = 311.3 × 333/288 = 359.9 kPa
- P₂(gauge) = 359.9 − 101.3 = 258.6 kPa
That is about 9 kPa above the 250 kPa fuse limit, so the 210 kPa service figure is not compatible with the 250 kPa limit if the tyre really reaches 60 °C. Before acting, confirm the manual revision and the design tyre temperature - the two published numbers do not sit together.
2. What service pressure at 15 °C would keep the hot pressure at or below 250 kPa gauge? What action follows, and how does the earlier 262 kPa reading fit in?
Read the worked answer
Reverse the calculation: P₂(abs, max) = 250 + 101.3 = 351.3 kPa.
P₁(abs) = 351.3 × 288/333 = 303.9 kPa → P₁(gauge) = 202.6 kPa.
So the tyre should be serviced at about 200 kPa gauge, not 210 kPa, unless the manual's own figures are shown to be mutually consistent.
The earlier 262 kPa reading is not a contradiction - it confirms the physics. A tyre serviced at 210 kPa gauge and heated to roughly 60 °C must read about 259 kPa; the reading was real, the sign-off was the error. Action: raise a technical query against the manual and use the corrected figure in the meantime.
3. Check the −30 °C case. Is bead seating at risk, and should the tyre be topped up on the cold ramp?
Read the worked answer
Cold soak: T = 243 K.
P(abs) = 311.3 × 243/288 = 262.7 kPa → gauge = 161.4 kPa.
That is above the 150 kPa minimum, but with only ~11 kPa of margin; any leakage or a colder overnight soak drops it below, so the tyre must be checked before dispatch rather than assumed good.
Do not top up to 210 kPa gauge at −30 °C. Setting P(abs) = 311.3 kPa at 243 K returns to 311.3 × 288/243 = 369 kPa abs = 268 kPa gauge at 15 °C - 18 kPa over the fuse limit.
Conclusion: pressure may only be corrected for temperature, never set to the nominal figure at an off-nominal temperature. Servicing at 210 kPa on a cold ramp would plant an over-pressure that only shows up hours later.