When you’re piloting, it’s important to know how high you are in the sky, and this is where the altimeter comes in. But, as you may already know, the outside temperature can do strange things to your altimeter readings.
So in this article, we’re going to go through how an altimeter works, exactly how variations in temperature can affect the altimeter, why it does so, and what to do when it does.
We’ll also cover some of your most frequently asked questions on the subject along the way.
How Does An Altimeter Work?
The basic principle behind an altimeter is pretty simple – it uses a thermometer to measure air pressure at different heights above sea level. The higher up you are, the lower the air pressure will be.
This is because the air molecules move faster as they get closer to the ground, due to gravity, which means that there’s less mass of air between them and the surface.
So if you were standing on top of Mount Everest, say, for example, the air would be moving much more quickly than it was at sea level.
And since air pressure is directly related to its density (the amount of matter per unit volume), then the air pressure will decrease with height.
This is measured by using a barometric sensor, or simply put, a ‘pressure transducer’. It converts changes in air pressure into electrical signals (see also “Does Bluetooth Work On Airplanes?“), which are then sent to the altimeter display to give you a read-out.
Why And How Do Altitude Readings Change With Temperature?
As we’ve said earlier, air pressure decreases the higher you are. This is caused by two factors; firstly, the speed of the air molecules increases as they approach the Earth’s surface.
Secondly, the air particles have less space between each other as they get closer to Earth.
But the air doesn’t just stay still when it gets close to the surface. It actually moves around quite a bit, which causes the air pressure to fluctuate slightly over time.
When the air is warmer, it expands more easily, meaning that the air pressure drops slightly.
But why does it drop? Well, the air is expanding due to the heat from the sun. If the air is cooler, it contracts more easily, causing a greater amount of matter per unit volume area, so the air pressure read out will rise slightly.
So now you know how the altimeter works, let’s look at some examples of how it might behave.
Example 1: Altitude reading at 25°C vs 10°C
Let’s say you’re flying along at 5,000 feet above sea level, and the air temperature is 25°C. At this point, the altimeter should read about 2,500 ft.
Now, let’s say that the air temperature suddenly drops to 10°C. Since colder air has a larger volume, it will expand more easily, making the altimeter show a higher reading.
In fact, the altimeter will probably register somewhere between 3,000 – 4,000 ft.
Example 2: Altitude reading at 50°C vs 15°C
Now, imagine that you’re flying along at 6,000 feet above sea-level, and the air temperature suddenly jumps to 50°C.
As the air heats up, it expands more easily. But since the air is also getting thicker, the altimeter will record a lower reading.
In fact, the altimeter may even register below 5,000 ft.
What Should I Do If My Altimeter Shows A Wrong Altitude Reading?
If your altimeter is showing a wrong altitude reading, it could mean one of three things:
- Your aircraft isn’t level
- You’re not flying at the correct altitude
- There’s something wrong with the altimeter itself
The best way to check whether your aircraft is level is to use an electronic compass.
An electronic compass measures the direction of magnetic north, so if it indicates that your aircraft is tilted, then you need to make sure that you’re flying straight and level.
To check whether you’re flying at the right altitude, either measure the distance to the horizon, or compare your current position to the map.
Finally, if you suspect that there’s something wrong with the instrument, contact your local FBO for advice. It may be that the altimeter is calibrated incorrectly.
How Far Off Can Your Altimeter Be?
It depends on what type of altimeter you have. Most modern instruments are accurate within ±10ft/m (±0.5 m).
However, older models may only be accurate to ±20ft/m (±1 m), and those used in military aircraft may be inaccurate to ±50ft/m (±2 m).
Can An Altimeter Be Adjusted For Temperature?
Yes, we are pleased to confirm that an altimeter can be adjusted for temperature. However, this can only be done at the factory.
The manufacturer can adjust the altimeter for changes in temperature, but you’ll need to take your aircraft back to them to make the adjustment.
Does Temperature Affect Airspeed?
No, we can confirm that temperature does not affect airspeed. Airspeed is measured using pitot tubes (or static ports), which detect the speed of the surrounding air.
How Does Temperature Affect Density Altitude?
Temperature affects density altitude by changing the size of the molecules in the air. This means that the density altitude will change as the temperature changes.
However, the density altitude is only used to calculate the true airspeed, so it won’t affect the actual airspeed on your display (see also “How Does Heads Up Display Work“).
What Effect Does Colder Than Standard Temperatures Have On True Altitude Compared To Indicated Altitude?
Colder than standard temperatures can cause the altimeter to give a false indication of altitude.
If you’re flying at 5,000ft AGL, and the air temperature dips to -10 °C, the altimeter will show a reading of around 7,500ft.
This is because the air becomes denser when it gets cold, meaning that it takes longer for the altimeter to read out its height.
So, there we have it. The temperature of the air affects air pressure, and since an altimeter uses a thermometer and a pressure transducer to give you your altimeter readings, you may not always get a true reading.
It’s important to be aware of this when flying, so that you can compensate accordingly.
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