Navigation - Lesson Three
- -Vertical: Altitudes and Flight Levels- -

Altitude Measuring:

Ever wondered why ATC gives you an altimeter setting upon departure or arrival to an airport? In order to explain this we must first provide you a little theory of altitude measuring by a device called barometric altimeter. As away we go from the surface the air gets thinner and its pressure gets lower. Actually, the altitude is proportional to the pressure. Our altimeter measures outer air pressure and calculates the altitude belonging to that pressure. The only problem is that there are other factors influencing air pressure, like whether and temperature. In order to assure that everyone is flying at the altitude they're supposed to be, we need to adjust our altimeters to variations in the atmospheric pressure.

There are several types of altitude you need to be aware of as you fly:

• Indicated Altitude - The value shown on your barometric altimeter whatever pressure it is set to.
• Pressure Altitude - Read on your altimeter when it is set to 29.92 inches or 1013 millibars that's called the Standard Athmospheric Pressure (QNE). This is the altitude which all performance figures are based upon and this is what we generally use during the enroute part of flight.
• True Altitude - Your actual height above sea level. If you set your altimeter to the local QNH (see below) it will show your True Altitude. This is what we use during departures and arrivals as well as for VFR flying.
• Absolute Altitude - Your actual height above terrain. If you set your altimeter to the airport QFE (see below) it will show your Absolute Altitude while overflying the airport. This altitude is used to determine decision height for ILS approaches and sometimes for over water navigation.
• Radio Altitude - Same as Absolute Altitude, but in this case a small transceiver is used to measure height above the surface, showing your exact height wherever you fly, provided you are below 2000 feet AGL.

The above altitude settings are used by the ATC to adjust our altimeters for variations in atmospheric pressure and to standardize all altimeters for high altitude flight. In this manner it can be assured that everyone is flying at the altitude they're supposed to be, that's important for maintaining separation between one aircraft and another as well as between the aircraft and ground obstacles.

Standard Pressure (QNE):

QNE or STD altitude is used for high altitude flying where aircraft speeds are high and ground obstacles are not an issue. The Standard setting allows pilots to complete their enroute flight without having to twist the altimeter knob every 5 minutes to set one local QNH after another. In addition it assures that all aircraft above a certain altitude use the same altimeter setting. Height above this so-called "transition altitude" is measured in Flight Levels, where 1 Flight Level equals 100 feet.

Now, when the ATC instructs to climb FL200, an experienced pilot like you just sets the altimeter to 29.92 inches or 1013 millibars, then starts climbing to 20,000 feet, and asks for the refreshments.

Sea Level Pressure (QNH):

QNH that's usually provided by an ATIS or ATC station is the barometric pressure reading of a given point on the surface, reduced to sea level. By setting the local QNH on your altimeter you can assure that the same altimeter reading represents the same height above ground in every weather situation. This is very important when you fly close to the ground, where incorrect altimeter setting may result in hitting a hilltop or any other ground obstacle. Most Approach Charts suppose that your altimeter is set to the local QNH.

When you are given a QNH, just read back and set your altimeter to the required setting. But they gave me a setting that doesn't fit on my altimeter! Usually this happens with US pilots flying in Europe or vice versa. In the US the setting is in inches of mercury, while Europeans use the hectopascal or millibar. Have no fear, you can make an easy conversion with a handy calculator. The formula is: 1 HPa = 2.9536 in or 1 in = 0.3386 HPa.

Surface Pressure (QFE):

QFE, that is sometimes used as an alternative to QNH, is the barometric pressure on the surface. If you set your altimeter to QFE, it will read Zero when you stand on the Runway. An altimeter set this way will always show your true height above the Runway. This is to the contrary to the QNH setting, where your altimeter reading will equal the published airport Elevation when you are on the ground.

When an ATIS station or a friendly Controller gives you a QFE, there's again nothing to get worried about. It's just another altimeter setting that gives you a reading relative to the ground. If a QFE setting seems inappropriate, you can make a conversion using the familiar formula: 1 HPa = 2.9536 in or 1 in = 0.3386 HPa.

Transition Altitude:

The altitude where you should switch from STD pressure to the local QNH / QFE or vice versa is called "transition altitude". To make things complicated, this transition altitude is different for each country! In the US, it is 180,000 feet or FL180. The below table shows the transition altitudes in several European countries:

While the above transition altitudes are effective all over the country, different transition altitudes may be in force within the controlled airspace around major airports. Such transition altitudes are shown on the Approach Charts.

Cruise Altitude:

Another important aspect of vertical navigation is choosing a cruise altitude. We cannot simply choose any altitude that seems economical. When flying Eastbound (on a track of 0 to 179 degrees) we always use odd thousand feet altitudes (called Odd Levels). When flying Westbound (on a track of 180 to 359 degrees) we use even thousand feet altitudes (called Even Levels). This way, at least 1000 feet vertical separation is maintained between aircraft flying in opposite directions on the same airway.

Above a certain altitude, however, the required vertical separation increases to 2000 feet, so Even Levels are no longer used, but every second Odd Level is allocated for Westbound flights instead. The changover altitude is 29,000 feet for NVSM (Normal Vertical Separation Minima) and 41,000 feet for RVSM (Reduced Vertical Separation Minima) operations that's effective over the most of Europe and the North Atlantic). The below table shows the Flight Levels to be used under NVSM and RVSM rules:

From the above examples, you can easily calculate the nearest suitable flight level for any target altitude.

Example Flight: Dublin - Heathrow using SIDs and STARs in ATC environment

• Set the local QNH for Dublin that we've got to know from the ATIS info.
• Taxi to Rwy, takeoff and follow SID routeing, climbing initially to 5000ft /QNH.
• At transition altitude (5000ft) set the Standard Pressure (29.92 in / 1013 HPa).
• Continue climbing to our assigned cruise altitude that is FL310 when cleared.
• Fly the STAR routeing to BNN, descending to FL70 or as instructed by ATC.
• Continue descending to 2500ft /QNH, following the vectors for the approach.
• At transition altitude (6000ft) set the QNH for London that the ATC gave us.
• Fly the approach (provided we have landing clearance from Heathrow Tower).
• Decide to land or go-around at Cat II decision height of 73 ft Radio Altitude.
• Land, then clear the active Rwy and taxi to the assigned terminal gate.

Now, you know enough about altitudes to impress controllers. To make them really happy, you should just pay a little attention to choosing a valid cruise altitude, following the climb and descent profiles prescribed in your SID/STAR, and be always where you should be - even in the vertical.