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A.FLOOR
4th Feb 2004, 10:28
Could someone please explain why we add height corrections to the MRA'S during strong winds?

What effect does the wind have on the MRA ?

Stickies
4th Feb 2004, 17:12
Excuse my ignorance but, if an MRA is the same as an MSA, I have always thought that the increase was to counter the effects of down draughts, standing wave effects and localised reduced pressure (especially on the lee side of high ground).

m&v
5th Feb 2004, 02:35
stickies,I think he means MEA's(min Enroute Alt')..MRA's are the MIN Reception Alt's for Nav-aids..Higher than Mea's..I concur 'wave' effect would require a higher Mea in areas of known turb' and downdraft over the rocks..:O

and MSA's only cover the 25miles from the charted fix...

alf5071h
5th Feb 2004, 05:05
The basics are given in ICAO publications: PANS-OPS pt6 chap3? Check for latest revision. N.B my italics; the thread answer is at the end.

PRESSURE CORRECTION
Flight levels. When flying at levels with the altimeter set to 1013.2 hPa (2992 in), the minimum safe altitude must be corrected for deviations in pressure when the pressure is lower than the standard atmosphere (1013 hPa). An appropriate correction is 10 m (30 ft) per hPa below 1013 hPa. Alternatively, the correction can be obtained from standard correction graphs or tables supplied by the operator.
QNH/QFE. When using the QNH or QFE altimeter setting (giving altitude or height above QFE datum respectively), a pressure correction is not required.

TEMPERATURE CORRECTION
Requirement for temperature correction. The calculated minimum safe altitudes/heights must be adjusted when the ambient temperature on the surface is much lower than that predicted by the standard atmosphere. In such conditions, an approximate correction is 4% height increase for every 10 C below standard temperature as measured at the altimeter setting source. This is safe for all altimeter setting source altitudes for temperatures above -15 C
Tabulated corrections. For colder temperatures a more accurate correction should be obtained from Tables. See other PPrune Threads

MOUNTAINOUS AREAS EN-ROUTE
The MOC (min obstacle clearance?) over mountainous areas is normally applied during design of routes and is stated in State aeronautical information publications. However, where no information is available, the margins below may be used when:
a) the selected cruising altitude or flight level or one engine inoperative stabilizing altitude is at or close to the calculated minimum safe altitude; and
b) the flight is within 10 NM of terrain having a maximum elevation exceeding 3 000 ft.

Margin in mountainous areas
Terrain variation: MOC
Between 3000 ft and 5000 ft: 1476 ft
Greater than 5 000 ft: 1969 ft

MOUNTAINOUS TERRAIN - TERMINAL AREAS
The combination of strong winds and mountainous terrain can cause local changes in atmospheric pressure due to the Bernoulli effect. This occurs particularly when the wind direction is across mountain crests or ridges. It is not possible to make an exact calculation, but theoretical studies (CFD Norway, Report 109.1989) have indicated altimeter errors as shown below. Although States may provide guidance, it is up to the pilot-in-command to evaluate whether the combination of terrain, wind strength and direction are such as to make a correction for wind necessary.
Altimeter error due to windspeed
Windspeed (kt): Altimeter error (ft)
20 : 53 ft
40 : 201 ft
60 : 455 ft
80 : 812 ft
Note. The windspeed values were measured 30 m above aerodrome elevation.

Corrections for windspeed should be applied in addition to the standard corrections for pressure and temperature, and ATC advised.