There are certain errors associated with all altimeters. What you need to be concerned with as a pilot is understanding that when flying in cold air, even with the correct altimeter setting, the altimeter WILL indicate higher than you really are. This is a big problem when trying to navigate your way around terrain and especially towers. This is such a problem in fact, that some northern airports actually have notes on the approach plate with higher minimums that apply when the temperature is below a certain level.
This altimeter does decrease at lower altitudes, so one of the most dangerous scenarios may be trying to avoid cell phone towers built on the top of a mountain at night, and trying to fly above them. You may not have as much vertical clearance between you and the towers as you think. How big of a problem is this really? Take a look at the chart on the right to get an idea of how far off your altimeter may actually be based on your altitude and temperature outside. (note: no need to memorize this chart, just know that it exists and know how to find and reference it before you go flying in really cold weather).
REMEMBER THIS: High to Low Look out Below! Low to High Clear the Sky! This applies both to flying from an area of high to low pressure, as well as flying from an area of high to low temperature, and vice versa. If you fly from an area where the altimeter setting is 30.20 inches, to an area where the setting is 29.90 inches, and do not adjust the altimeter setting in your “kollsman” window, you will be flying 300′ lower than your altimeter shows you are.
Typical “Rules of Thumb” for altimeters are:
- 1″ of pressure equals 1,000′ (i.e. you have the altimeter set to 29.92 and the local setting is 30.42, you are a half inch off, thus you are flying 500′ higher than indicated)
- Flying in cold air your altimeter will indicate that you are higher than you really are
- Flying from an area of high pressure to low pressure is dangerous if you don’t reset the altimeter with the new local setting often
- You must reset your altimeter to the new local setting at least every 100 miles along your route of flight (more often the better)
Five Different Types of Altitude
Okay, full disclosure, what we are about to talk about is really more pertinent to the written exam than actually flying an airplane. At this point in your flying career, you are probably familiar with three types of altitude at least (Density altitude, MSL, and AGL). Those three types can easily keep you alive while flying for many, many years. Since we want to get a great score on the written exam though, let’s go ahead and define the 5 types of altitude the FAA wants you to know about.
- True Altitude: You care about the truth right? Well this is the altitude that is truly important, it is just another way of saying MSL or altitude above mean sea level. This is the altitude you are generally reading right from your altimeter.
- Indicated Altitude: Whatever altitude your altimeter is indicating at that particular time. It’s just what the altimeter is showing.
- Absolute Altitude: This is the altitude that is ABSOLUTELY important to you, it is your height AGL or above ground level (it matters how high you are above terrain right?)
- Pressure Altitude: Set the altimeter to 29.92 and read what it says, this is your pressure altitude. The lower the local altimeter setting the higher the altitude your altimeter will indicate.
- Density Altitude: This is pressure altitude corrected for non-standard temperature. In simpler terms, this is the “Feels Like” altitude (just like it might be 90 degrees in the summer in Florida, but feels like 105 degrees). Density Altitude is the altitude that the airplane “feels like” it is at, and it will perform accordingly. The higher the density altitude, the thinner the air is, and the worse the airplane will perform. (less air for the engine to make power, and less air for the wing to generate lift).
Many airplanes will have the static port combined with the pitot tube or pitot vane. Others may have just a single, or dual static port located elsewhere on the aircraft.
Some airplanes may also be equipped with an alternate static source that will take a “static air pressure” reading from inside the cockpit (or somewhere where the port would never become blocked in-flight by ice or any other means). When using “alternate static air” also known as “alternate static source”, it is important to remember that due to the way the air flows around the airframe in-flight, there will always be slightly less air pressure inside the cockpit. This means there will be some errors associated with using “alt static air” and you’ll need to remember these:
- Airspeed will Indicate Higher than Actual
- Altimeter will Indicate Higher than Actual
- VSI will momentarily indicate a climb when “alt static air” is activated
Generally, you will activate the alternate static source by pulling a small knob on the instrument panel or turning a small valve somewhere near the instrument panel (your CFII can show you exactly where on your specific airplane). The reasons you will experience the errors listed above is this: when static pressure decreases around the airplane (i.e. you climbed so atmospheric pressure drops), you will have your altimeter showing higher than it should, and your VSI showing a climb when the pressure is actively decreasing. The airspeed works by comparing the “ram” air pressure against the static air pressure and moving the diaphragm accordingly. When there is less static pressure, but the same ram air pressure (like the airplane is still going at the same speed through the air, but you open the “alt static source” decreasing the pressure on the backside of the airspeed indicator diaphragm, the airspeed needle is going to show a higher airspeed than you are actually traveling at.
We are all familiar with Magnetic Variation (the isogonic lines we use on sectional charts to find the difference between true and magnetic north for our current location). What may be new to you as an instrument pilot is trying to remember the turning errors associated with the magnetic compass. While it may seem like a lot to remember, you may find yourself navigating one day after everything else in your airplane has failed, just by the magnetic compass, and the skill of flying by just the compass may save your life. We’ll make it as simple as possible for you to remember exactly what you need to know to fly the airplane, and pass the written exam below:
ANDS (Accelerate North, Decelerate South)
When on an EAST or WEST heading, if you accelerate, the compass will momentarily show a turn to the north even when your heading is actually staying constant. There is not a turning error when accelerating or decelerating on a north or south heading.
OSUN (Overshoot South, Undershoot North)
When turning to a Southerly heading the compass will lag. Everything moves slower in the South right? (LOL) If you’re trying to roll out on a Southerly heading, overshoot the heading (rollout later) and you’ll probably be where you want to be, you’ll see the compass catch up in 10-15 seconds. Basically, OVERSHOOT the heading you’re turning to when turning southbound.
When turning to a Northerly heading the compass will turn faster than you are actually turning. Those Yankees move fast don’t they?! So undershoot the heading you are trying to rollout on. I.e You are turning towards a heading of 030 (doing a standard rate turn), from a heading of 180. As you see 050 or 045 on the compass, start your rollout, you’ll probably wind up right on 030.
You can bank on these errors everytime!
(A) When on a North heading and you bank to the left, the compass initially indicates a turn to the right. And when you bank right it initially shows a turn to the left.
(B) When on a South heading and you bank to the left, the compass indicates a turn to the left (but initially at a faster rate than is actually occurring). And when you bank right it initially shows a turn to the right at a faster rate than is actually occurring.
I remember this in terms of Barbeque. Nobody beats good southern BBQ. The phrase to remember here is: “North lags and South leads” Starting a turn from a north heading the compass will lag, starting a turn from a south heading the compass will lead.
You’ve probably seen one of these in every airplane you’ve ever flown. That’s because it is legally required in every airplane regardless of flying IFR or VFR. If the airplane is missing this attached to or near the magnetic compass, then it’s a no-go on flying until a maintenance tech can come out and do the test to write up a new one.