Altimeter settings & transitions

What is the datum below the aircraft?

Distinguish a geometric distance from a pressure-derived indication.

Altitude, height and elevation

Altitude is the vertical distance of a level, point or object considered as a point, measured from mean sea level. Height uses a specified datum, such as an aerodrome or the ground immediately beneath the aircraft. Elevation is the vertical distance above mean sea level of a point on or attached to the Earth's surface. These terms specify the reference from which a distance is measured; they are not interchangeable labels for any altimeter number.

True altitude is the aircraft's actual geometric vertical distance above MSL. True height is its actual vertical distance above the specified datum. Terrain clearance uses the terrain beneath the flight path: true altitude minus terrain elevation at that position. A height above an aerodrome does not automatically equal clearance above a hill some distance away.

An indicated altitude comes from the pressure altimeter's calibration and chosen setting. It agrees with true altitude only to the extent that the assumed pressure and temperature structure matches the real atmosphere, after allowing for instrument and other errors. A radio altimeter uses a different measurement principle and reference; it is not a substitute for correctly interpreting a pressure altitude in an ATPL meteorology calculation.

What changes when you turn the subscale?

Keep the aircraft stationary while changing the reference used by the instrument.

Three settings, three purposes

QNH is the altimeter subscale setting that makes a pressure altimeter indicate the aerodrome elevation when it is on the relevant aerodrome datum, within the applicable errors. In flight, QNH gives an indicated altitude referenced to MSL through the altimeter's standard calibration. It does not remove temperature error through the whole air column. QFE is pressure at the specified aerodrome datum; with the appropriate QFE set, the indication is approximately zero at that datum and height above it in flight.

The standard setting is 1013.25 hPa, approximately 29.92 inHg. On this setting the indication is pressure altitude: the altitude assigned to the measured pressure by the standard atmosphere. A flight level is a surface of constant atmospheric pressure related to this standard datum and separated from other such surfaces by specified pressure intervals. FL80 means the standard-pressure indication is 8,000 ft; it does not guarantee 8,000 ft true altitude.

At a fixed static pressure, increasing the subscale setting increases the indicated altitude; reducing the setting reduces it. The aircraft has not physically moved. This direction check is essential before doing the arithmetic. An incorrect pressure setting can produce a clearance error even in standard temperature, while a correct QNH can still leave a temperature-related true-altitude error aloft.

When does an altitude become a flight level?

Distinguish the published geographical boundary from the pressure-dependent descent boundary.

Transition altitude, level and layer

The transition altitude is the altitude at or below which the aircraft's vertical position is controlled by reference to altitudes. It is promulgated for the area or aerodrome; do not assume one universal EASA value. The transition level is the lowest flight level available for use above the transition altitude. Its selection accounts for pressure so that the required transition-layer depth is maintained. The transition layer is the airspace between the transition altitude and transition level.

During a climb through the transition altitude, change from the applicable QNH reference to the standard setting according to the published and ATC procedures, and thereafter express vertical position as a flight level. During descent through the transition level, change to the applicable QNH and express it as an altitude. The same aircraft at the same instant changes its indicated number when the subscale is changed; its true position does not jump.

The transition layer is generally used for the reference change during climb or descent rather than normal level cruising. A clearance, a restriction and a pressure setting must be read together. The transition level can change as QNH changes; the transition altitude is not simply relabelled FL50 whenever pressure is low. The local AIP and current ATC information determine the applicable procedure, including any use of QFE.

Why is the lowest numbered flight level not always usable?

Combine pressure, temperature, terrain and the available flight-level scheme.

A pressure level still needs physical clearance

A low QNH lowers a given standard-pressure surface relative to its height in standard sea-level pressure. Cold air reduces the geometric thickness between the setting datum and the aircraft. Together, low pressure and low temperature can place an aircraft at a flight level significantly lower in true altitude than its standard indication suggests. Terrain clearance must be checked in the actual column, not inferred from the flight-level number alone.

The lowest usable flight level must satisfy the applicable minimum altitude or terrain-clearance requirement under the pressure and temperature conditions, while also respecting the levels available for the route and direction of flight and the transition arrangements. A calculation can establish a minimum pressure altitude, but the next mathematically higher hundred feet is not automatically an assignable cruising flight level.

The required minimum clearance is supplied by the applicable procedure, chart or examination question. Do not invent one universal obstacle margin for all operations. In a worked problem, first find the true altitude needed above the stated terrain, convert that requirement to an indicated altitude allowing for temperature and pressure, then select a permissible flight level from the available choices. Real operation uses approved altitude and cold-temperature procedures as applicable.

Can an altimeter over-read near a mountain?

Distinguish local airflow pressure effects from the broad temperature-column error.

Local static pressure can be lower than expected

Air forced through a constriction or accelerated over a ridge can experience a local reduction in static pressure. A pressure altimeter interprets lower static pressure as a greater altitude, so its indication can over-read the aircraft's true position in this situation. The relevant link is acceleration, lower static pressure and an altitude indication that is too high.

This Bernoulli-type effect is a qualitative model, not permission to calculate a safe mountain clearance from wind speed alone. Real mountain airflow is three-dimensional and may contain waves, rotors, separation and strong vertical currents. Local pressure anomalies and downdraughts can occur together. A static-system position error is another distinct source of error; the mountain-flow effect concerns the air's actual local static pressure.

Broad low pressure and cold-temperature effects are addressed separately with altimeter-setting and temperature corrections. Setting an up-to-date QNH does not guarantee that all local airflow pressure effects disappear, and a correctly indicated pressure altitude does not reveal a hidden downdraught. Use appropriate terrain, wind and mountain-flight limitations rather than treating an altimeter reading as a direct terrain sensor.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.