# How much does an airliner cabin elongate at altitude?

How much (about) is a commercial plane's cabin elongated at cruising altitude, due to interior-exterior pressure difference?

Of course there should be a slight shrinkage because of temp difference, but I estimate that overall a 30m cabin should be elongated by few centimeters.

• Concorde excluded? Aug 29 '20 at 1:51

## 1 Answer

We can identify three effects that have an effect on cabin length:

• Stretching due to the bulkhead pulling on the end of the fuselage;
• Shortening due to the fuselage expanding in diameter (Poisson effect)
• Shrinkage due to temperature

These are simple solid mechanics problems. Given an airplane with length $$l$$, diameter $$d$$, wall thickness $$t$$ made with a material with Young's modulus $$E$$ and Poisson ratio $$\nu$$ and linear expansion coefficient $$\alpha$$ pressurised to a pressure differential $$p$$, cooled to a temperature differential $$\Delta T$$....

$$\Delta L = \dfrac{d p L}{ 4 t E} - \dfrac{\nu d p L}{2 t E} - \alpha \Delta T l$$

$$\Delta L = \dfrac{d p L(1-2\nu)}{ 4 t E} - \alpha \Delta T l$$

Let's have a wall thickness of 3mm, diameter of 4m, length of 40m subjected to 0.5bar and 70°C temperature difference. Aluminium $$E=$$70GPa, $$\nu=0.33$$, $$\alpha=25\mu m/m°C$$. Filling in the numbers, we obtain $$\Delta L=3.2mm-70mm\approx-67mm$$

So thermal expansion definitely has the upper hand here, shrinking the fuselage by a few centimetres in total, whereas pressurising the fuselage can only expand it by a few millimetres. The numbers here are very much approximate but the order of magnitude will not change much with "real" numbers.

• I wonder if someone did actually made a measurement. It's very easy using a laser meter. Aug 29 '20 at 8:49
• I'm not sure airline operators will appreciate it if you shine a laser through a cabin full of passengers. Aug 29 '20 at 9:03
• Interesting... I'll love to know what's the Poisson's effect contribution... Aug 29 '20 at 10:51
• @xxavier To cover that exact question I already wrote the equation to easily show the Poisson effect, namely, the $(1-2\nu)$. So we have about 10mm extension minus about 7mm Poisson contraction. Aug 29 '20 at 11:59
• Since the elongation of the cabin due to the pressure difference is so small, I wonder how this miniscule difference can cause metal fatigue to the airframe. The length change due to temp difference that is much more impotant, is inherent to the metal, it should not provoke metal fatique. Am I wrong? Aug 29 '20 at 14:56