After the 2010 eruptions of the Eyjafjallajökull vulcano in Iceland, air traffic (including general aviation) was suspended for days in large parts of Europe. Even glider aircraft were initially not allowed to fly: authorities were afraid that the static ports of the glider would get blocked by the ash, which would cause the altimeter to remain at zero altitude and the speed indicator to display incorrect values.

Question

Consider a glider flying at 400m, when all of the sudden the static ports get blocked due to volcanic ash. The pressure at this altitude is 96610 Pa. It then descends to sea level where it flies at a true airspeed of 25 m/s.

What is the airspeed indicated by the pitot-tube driven airspeed indicator in m/s?

Answer 1

To determine the indicated airspeed (IAS) when the static ports are blocked in the glider, we need to analyze how the airspeed indicator works when there is a blockage.

When the static ports are blocked, the static pressure reading remains constant at the last measured value, which in this scenario is the pressure at 400 m altitude (96610 Pa). As the glider descends, its dynamic pressure will change according to the new altitude (sea level). However, the airspeed indicator will continue to read based on its last static pressure measurement.

1. Calculate the dynamic pressure at sea level:

The dynamic pressure (q) is calculated using the formula: \[ q = \frac{1}{2} \rho V^2 \]

where:

• \(\rho\) is the air density,
• \(V\) is the true airspeed.

At sea level, we can assume the air density (\(\rho\)) to be approximately \(1.225 , \text{kg/m}^3\) for standard atmospheric conditions.

2. Calculate the dynamic pressure at sea level with an airspeed of 25 m/s: \[ q = \frac{1}{2} \cdot 1.225 \cdot (25)^2 \] \[ q = \frac{1}{2} \cdot 1.225 \cdot 625 \] \[ q = 0.6125 \cdot 625 \] \[ q = 383.125 , \text{Pa} \]

3. Now calculate the indicated airspeed (IAS) using the blocked static pressure:

When the static ports are blocked, the indicated airspeed is calculated using: \[ q = P_{static_blocked} - P_{local} \] Where \(P_{static_blocked}\) is the pressure at altitude (96610 Pa) and \(P_{local}\) at sea level is approximately 101325 Pa.

The local pressure (at sea level) minus the static pressure at 400 m gives: \[ P_{local} = 101325 , \text{Pa}, \quad P_{static_blocked} = 96610 , \text{Pa} \] \[ q = 96610 - 101325 = -4650 , \text{Pa} \]

Since negative pressure doesn't have physical meaning, we assume the displayed value correlates to the change in dynamic pressure applied against the constant pressure outside the aircraft.

This would give us a relationship: Using the earlier mentioned dynamic pressure formula to find the indicated airspeed (IAS): \[ q = \frac{1}{2} \rho \cdot IAS^2 \]

Rearranging for IAS gives: \[ IAS = \sqrt{\frac{2q}{\rho}} \]

Since at sea level for a true airspeed of 25 m/s, there was a disagreement, we essentially track the quirk wherein IAS doesn't correspond correctly to the dynamic value due to blocked static reading which is wrong.

While there is an inherent difficulty in calculation due to air indicators, given a true speed of 25 m/s is established: The true airspeed measured indicates "invalid" in the blocked state, generally reflecting zero above the critical. Hence the indication would falsely stay zero as actual atmospheric feedback changes remain unmeasured.

The direct loss in indication to reflect IAS turns simply back to zero rather than retrieving the effective number under an assimilation of atmospheric values.

Thus, if we were to reflect against standard operation ideals, the IAS during the inhibited block state would indeed reflect falsely to zero m/s indicating erroneous and non-functional static port states during volcanic ash incursion.