Determine the lowest usable flight level for an IFR flight provided that the MOCA is 3 300 ft and the transition altitude is 3 000 ft, given:

QNH: 1020 hPa

Temperature: -7°C

The airport elevation is: 27 ft

Magnetic course: 182°

Variation: 8°E

__Calculation of terrain clearance and lowest usable flight level:__

The aircraft must fly at __3 300 ft__ *(MOCA).** *This* *altitude must be the true altitude of the aircraft. This altitude must be corrected for any temperature ISA deviation and then any difference in pressure from standard (1013 hPa), to get the pressure altitude, allowing us to work out the lowest usable flight level.

To compute the temperature correction, it is necessary to have knowledge of the ISA air temperature. In the International Standard Atmosphere (ISA), the air temperature at sea level is 15ºC, and the temperature lapse rate is -2ºC/1 000 ft. The formula to calculate the ISA temperature is: ISA temperature = 15ºC - (27/1 000) x 2, giving an ISA temperature of approx. 15ºC. The actual temperature is -7ºC, which is an ISA deviation of ISA -22ºC.

The first step is to determine the temperature correction. The ISA deviation is __ISA -22ºC__. For the temperature correction, the airfield elevation must be taken into account: 3 300 ft - 27 ft = 3 273 ft. Based on the 4% rule and its calculation mentioned in the rules below, the height correction for the temperature can be calculated as follows: Height __correction for temperature__ = 4 × (-22) × (3 273/1 000) = __288__ __ft__.

In this case, the temperature correction is **added** to the true altitude as per the rules below (refer to the table below), to give an indicated altitude of __ 3 588 ft__.

Next, we need to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa: 1020 hPa - 1013 hPa = __7 hPa__. Since the barometric lapse rate near the surface is 30 ft/hPa, the pressure correction can be calculated as follows: __Pressure correction__ = 7 hPa x 30 ft/hPa = __ 210 ft__. This value needs to be

**subtracted**from the indicated altitude as per the rules below:

**Pressure Altitude = 3 588 ft - 210 ft = 3 378 ft.**

According to the ICAO IFR Semi-Circular Rule, below FL290:

*For Easterly magnetic tracks (000° - 179°), select ODD FLs (FL110, FL130, etc.)**For Westerly magnetic tracks (180° - 359°), select EVEN FLs (FL100, FL120, etc.)*

**south magnetic track**of 182º(M). Thus, the Minimum Flight Level, which provides obstacle clearance, after both pressure and temperature correction, that must be selected is:

__.__

**FL40****The following rules should be considered for altimetry calculations:**

__RULES.__- All calculations are based on rounded pressure values to the nearest lower hPa.
- The value for the barometric lapse rate between MSL and 500 hPa to be used is 30 ft/hPa as an acceptable approximation of the barometric lapse rate.
- To determine the true altitude/height, the following rule of thumb, called the ‘4 %-rule’, shall be used: the altitude/height changes by 4% for each 10°C temperature deviation from ISA.

*For simplification:**Height correction for the temp = 4 × (**ISA DEV**) ×**Indicated alt/1000**=**___ ft* - If no further information is given, the deviation of the outside-air temperature from ISA is considered to be the same throughout the whole layer.
- The elevation of the aerodrome has to be taken into account. The temperature correction has to be considered for the layer between the station (usually an aerodrome) and the position of the aircraft.

HIGHER PRESSURE; INDICATED ALTITUDE > PRESSURE ALTITUDELOWER PRESSURE; INDICATED ALTITUDE < PRESSURE ALTITUDE |
WARMER THAN ISA; TRUE ALTITUDE > INDICATED ALTITUDECOLDER THAN ISA; TRUE ALTITUDE < INDICATED ALTITUDE |

__DEFINITIONS.__*Pressure Altitude: The altimeter indication with standard pressure (1013.2 hPa) set.*

Indicated Altitude: The altimeter indication with local QNH set.

Indicated Altitude: The altimeter indication with local QNH set.

*True altitude: The actual altitude of the aircraft above mean sea level.*

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