Category: Aviation

SGR Calculation

Calculations of SGR are as follows;

consumption rate (lb/kg/etc per hr) / ground speed (kts) = lb/kg/etc per nm

OR;

ground speed (kts) / consumption rate (lb/kg/etc per hr) = nm per lb (or whatever you’ve used)

Consumption Rate

This is calculated by taking TAS and dividing by SAR (Still air range)

TAS / SAR (nm per lb) = lb per hour

OR;

TAS * SAR (lbs per nm) = l per nm

To calculate the vertical speed required to reach a destination with a given groundspeed and distance, use the following:

Vertical speed required (fpm) = (altitude to lose (ft) x groundspeed (kts)) / (60 x distance to go (nm))

To calculate the distance required for descending at a particular vertical speed & groundspeed, use the following formula:

60 x descent (fpm) / groudspeed (kts) = ft/nm
altitude to loose (ft) / ft/nm = distance required

aka: altitude to loose (ft) / (60 x vertical speed (fpm)) = distance required (nm)

Another way to look at is: if you need to loose 24,000ft, descending at 2000 ft/min, it will take 12 mins (24,000 / 2,000 = 12). With a groundspeed of 240 kts, it equates to 4 kts per min (240 / 60 = 4). 4 (kts per min) x 12 (mins) = 48 nm (distance required).

Thanks to:

http://forums.flightsim.com/vbfs/showthread.php?27140-Rate-of-Descent-Calculation
http://forums.vatsim.net/viewtopic.php?f=8&t=6861
http://www.atpforum.co.uk/showthread.php?t=9907

Pressure Height

Get the difference between your current QNH & the standard sea level QNH (1013.25) … eg. AD QNH = 1006, means there is a difference of 7 hPa, which equates to a 210 ft difference (calculated by taking the calculated 7 hPa and multiplying it by 30 (30ft difference per hPa)

Once you have 210 as the height difference, you add (if your QNH is less than standard sea level pressure), or subtract it if it’s higher (eg. if the QNH is showing you’re actually less than sea level pressure) it to your current elevation (eg. 3310 ft).

The overall formula can be shown as;

Pressure Height = Elevation + 30 * (1013 – QNH)

Eg. At an elevation of 3310, with QNH of 1006.

1. Pressure difference is 1013 – 1006 = 7 hPa difference.
2. We then get the pressure height difference in feet … 7 * 30 = 210
3. Then add it to our current height … 210 + 3310 = 3520 feet.

Density Height

Density altitude in feet = pressure altitude in feet + (120 x (OAT – ISA_temperature))

ISA_temperature = the ISA temp at the altitude you are at … calculated by using;

ISA Temperature: Temperature changes at the rate of 2 degrees per thousand ft (gets colder as you go up, and gets warmer as you descend). The standard sea level ISA temp is 15 degrees, so you will need to subtract the temperature difference from 15 degrees to get the ISA temp…

Eg. The aerodrome (AD) is 3310 feet above mean sea level (ASML), and the temp is 28 degrees.

1. The ISA temp at 3310 is 3310 / 1000 = 3.31 …. * 2 = 6.62 …. 15 – 6.62 = 8.38 degrees.
2. We then need to find out the temperature deviation from the norm = 28 (our temp) – 8.38 (ISA temp at our altitude) = 19.62 … (rounded to the nearest degree gives 20 degrees).
3. From here, we need to use the temp deviation to find out our density height. This is done by the following;
   ISA Temp * 120 (120 ft per degree) = 20 * 120 = 2400
4. Then add 2400 to our pressure height (3520 in this case .. see the above section for workings) which gives us 5,920 ft.

Reference: http://en.wikipedia.org/wiki/Density_altitude

Sample questions;

1. Why must you minimize the time used to check the pitot heater?
   1. To minimize the drain on the battery
   2. So as not to impare the working life of the heating element
2. Once airbourne, pitot heat should be switched on;
   1. Well before you enter known icing conditions
   2. On entering known icing conditions and left on
3. If static lines fully block, the altimeter will display the level of the blockage?
4. A/C lands at AD (elevation 1750′) with ALT subscale set to AREA QNH 1020 hPa. AD QNH at the time is 1025hPa. In this case, what will the ALT display in feet?
5. On a climb at a constant IAS, does air leaves both casing & capsule at the same rate?
6. An A/C’s sole static source is fully blocked whilst on a climb. If the A/C settles down to CRZ in an area of MOD to SEV turbulence at the published turbulence penetration speed then;
   1. The airframe could be overstressed
   2. There should be no risk of overstressing the airframe
7. Where globally will a magnetic compass function best?
8. What purpose does the compas deviation card serve?

Answers;

1. B – Heating element gets very hot in still air conditions
2. 2