Today we go back to basics! The bread and butter of a cockpit instrument panel or Primary Flight Display are the basic instruments. A big chunk of them work using air, the other chunk are mostly gyroscopic. The ones using air pressure are called pitot-static, which we are going to focus on. So how do Pitot-Static Flight instruments work exactly?
For the cadets coming straight out of training this might be an easy one. But interestingly it’s the veterans and experienced pilots here that might be in need of a brush-up! Ready? We’ll divide this article up in the following sections:
What are Pitot-Static Flight Instruments?
The instruments on board an aircraft that require air pressure are the Vertical Speed Indicator (VSI), the Airspeed Indicator (ASI) and the Altimeter. What specific pressures do they need? Let’s have a look.
An aircraft that is stationary on the ground is being influenced by the pressure that is all around us due to the atmosphere around our planet. The common term for this atmospheric pressure or static pressure.
Once the plane (or any object really) starts moving though, we experience a 2nd type of pressure. We feel the force of air hitting our aircraft. This is called Dynamic Pressure. These 2 combined are called total pressure.
Why is any of this relevant? Well, our instruments onboard that use air to function either need static pressure, total pressure, or both:
To know our airspeed, we need to know how much dynamic pressure is hitting our aircraft. We do this by using the Pitot Tube. A Pitot Tube collects incoming air and is able to relay this information to the airspeed indicator.
The problem here is that we simply cannot measure dynamic pressure on its own, as it always includes static pressure as well. These 2 combined are called the total pressure as we mentioned before. So how can we measure only dynamic pressure? We use a combination of a pitot tube and a static port to isolate dynamic pressure!
The pitot tube receives the total pressure (static + dynamic). The static port receives, well, the static pressure! We then simply remove the static pressure from the total pressure, which leaves us with the dynamic pressure by itself. How this is done exactly will be discussed further down.
What is important to realise is that a pitot tube should be positioned in such a way that the aircraft itself is not influencing the airflow around it, otherwise we would get inaccurate readings.
This is why you often find them on odd looking sticks at the front of the aircraft, where the air is undisturbed. The direction should be parallel to the incoming air at ‘normal’ flight attitude. But despite all of this, we will always have a certain amount of position error. We will also get manoeuvre-induced error, which is caused by pitch changes and causes a lag in accurate readings in the cockpit. This is one of the reasons why there’s often multiple static ports.
The readings can be transmitted via small pipes that carry the pressure to the instruments. However, in modern aircraft this is now all done by electrical wires taking electrical readings from the pitot tube to the instruments for further processing.
How does an Airspeed Indicator work?
As we mentioned before, if we want to know our airspeed we will have to ‘count’ the amount of dynamic pressure affecting our aircraft. We need both the static port and the pitot tube for the ASI to work.
Imagine a hollow chamber with connected to a static port. The pressure inside the chamber is the same as the static pressure outside.
Then we have a diaphragm that is connected to the pitot tube, so it has the total pressure inside of it:
The diaphragm will expand or compress based on the difference in pressure between the chamber, and the pressure inside the diaphragm. Basically we are creating a contest between total pressure and static pressure, each pushing into the diaphragm but from different sides.
This is a physical way of subtracting static pressure from total pressure. What’s left is dynamic pressure! All we need to do now is design the diaphragm is such a way so that it converts the expansion to the correct amount of airspeed. This is done via gears and linkages, which then feed the information to the displays in the cockpit!
The indication we get is not the same as our actual airspeed though, due to compressibility factors, instrument error, position error, density errors, and manoeuvre induced errors. The airspeed we read is called indicated airspeed (IAS).
If we correct IAS for position and instrument error however we get what’s called calibrated airspeed (CAS).
If we then take compressibility into account and correct CAS for that, we get Equivalent Airspeed (EAS). Compressibility error is considered relatively small for aircraft flying with a TAS of less than 300 kts. To give you a rough idea, an airspeed near the speed of sound would result in a correction of about 20 kts.
Finally, if we also correct the EAS for the fact that less air will be entering our instruments due to the lower density (density error), we are left with True Airspeed (TAS)! We covered Density and Density Altitude in a recent article.
How does an Altimeter work?
On to the next pressure dependent instrument: the altimeter. This instrument is a lot simpler compared to the airspeed indicator.
The chamber of an Altimeter is directly connected to the static pressure port, which means the pressure inside the chamber is the same as the atmospheric pressure outside the aircraft.
Then, we have something called an aneroid capsule. This is an air tight capsule which has a calibrated pressure inside of it that never changes (usually the standard pressure of 1013 HPa, or 29.92 Hg for our American friends), although some are calibrated differently.
This aneroid capsule is able to expand or compress, based on the difference in pressure between the chamber and the pressure inside of the capsule. If we fly really high, the static pressure is lower than standard pressure, so the capsule expands.
If we fly lower, the static pressure is higher than standard pressure, so the capsule gets compressed.
Finally, the movement of the capsule is converted into a reading on the altimeter display via gears and linkages. It is calibrated based on the International Standard Atmosphere (ISA).
How does a Vertical Speed Indicator work?
And then we have the VSI, the most confusing one to most. The VSI only uses static pressure, but it uses it in two different ways. First, we have a similar chamber as before with a diaphragm that is connected to the static port this time.
This input line of static pressure splits off though into another line that enters the chamber directly, but there’s a small difference. This line has something called a calibrated leak:
This calibrated leak only let’s through a small amount of air. This causes a delay for the pressure inside the chamber to change. Compare this to the pressure inside the diaphragm, which changes almost immediately to whatever the static pressure outside is.
So let’s look at an example. Imagine we are in level flight, the pressure inside and outside the diaphragm is 1000 HPa, so it won’t expand or get compressed, the VSI reads 0 as there is no difference between the chamber and the diaphragm pressure.
Now let’s look at the climb. By climbing, we enter air with a lower static pressure. This means that the air inside the diaphragm will immediately lower as well to whatever the ambient pressure is.
However, the chamber outside will still have some of the (higher) pressure in it from when we were in level flight, it can’t equalise as quickly as the air inside the diaphragm. This means the diaphragm will…? Get compressed! This compression is then converted by linkages and gears to indicate a climb.
For instance, let’s say that we are now entering air with a pressure of 980 HPa. The pressure inside the diaphragm quickly becomes 980 HPa. But the pressure in the chamber is only slowly creeping towards 980 HPa (it lags):
The pressure in the chamber is now higher than inside the diaphragm which causes it to be compressed. This compression is translated to a climb indication on the VSI display in the cockpit via linkages.
Then for the descent, we enter air with a higher pressure. The higher pressure will quickly fill the diaphragm, but will not immediately be in the chamber. This means the diaphragm will expand and cause a descend indication on the VSI display.
How do blocked ports affect Pitot-Static Flight Instruments?
We all hate these questions during the ATPL exams. But let’s try and make this as simple as possible. What happens to your instrument indications if one of the ports get blocked?
The Altimeter is the easiest one. Whatever the pressure is when the port becomes blocked is the pressure that will remain inside the chamber, which will freeze the altimeter indication.
The VSI should slowly return to zero depending on where inside the air pipeline the blockage occurs. But both ends being connected to the static port means that pressure will eventually equalise, resulting in a 0 readout.
Then we have the more interesting one, the ASI. If we stay level and the pitot tube becomes blocked, the readout will stay the same, even if we increase or reduce airspeed. This is because the static pressure remains the same and the pressure inside the diaphragm stays the same as well.
A blocked static port during the climb means that while the static pressure should be reducing, it isn’t. This means the chamber has a higher pressure than it should have, compressing the diaphragm more than it should be: it’s under-reading. The opposite is true for a descent.
When we are climbing and the pitot tube is blocked, it means that while the dynamic (and therefore total) pressure should be reducing, it isn’t. This means the diaphragm has a higher pressure than it should have, as it’s expanding the diaphragm more than it should be: it’s over-reading. The opposite is true for the descent!
To remember this (as it can be a bit of brain buster): the mnemonic is PUDSOD:
Pitot Blocked: Under-reads in Descent
Static Blocked: Over-reads in Descent
So there we have it, the pitot-static system and how it all works! Use PUDSOD if you’re about to do your exams, or if you’re flying and the airspeed reading does not make sense. We will cover other instruments in the future and go over gyroscopes as well.