wind speed corrections

[Allan]

Hi.
Out of curiosity, does your operations manual specify corrections to be made to MSA, MORA and MEA when flying in or over mountainous areas and the wind speed as more than 31 kts? We have it in our manual, but I haven't been able to locate where these figures come from.

Kind regards

[safetypee]

The following extract is from PANS-OPS Part VI ‘ALTIMETER SETTING PROCEDURES’

3.5 MOUNTAINOUS TERRAIN - TERMINAL AREAS
3.4 MOUNTAINOUS AREAS — EN-ROUTE
The MOC 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 in Tables VI-3-3 and VI-3-4 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.

Table VI-3-3. Margin in mountainous areas (SI units)
Terrain variation, MOC
Between 900 m and 1 500 m, 450 m
Greater than 1 500 m, 600 m

Table VI-3-4. Margin in mountainous areas (non-SI units)
Terrain variation, MOC
Between 3 000 ft and 5 000 ft, 1 476 ft
Greater than 5 000 ft, 1 969 ft

3.5.1 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 in Tables VI-3-5 and VI-3-6. 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.

Table VI-3-5. Altimeter error due to windspeed (SI units)
Windspeed km/h, Altimeter error (m)
37, 17
74, 62
111, 139
148, 247

Table VI-3-6. Altimeter error due to windspeed (non-SI units)
Windspeed (kt), Altimeter error (ft)
20, 53
40, 201
60, 455
80, 812

Note. The windspeed values were measured 30 m above aerodrome elevation.

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