The surfaces of constant
pressure are supposed
horizontal.
These surfaces are not
modified by terrain.
Figure B8 - Isobaric-pressure surfaces Aircraft fly in cruise at given pressure altitude, with a common pressure reference, which is agreed worldwide: 1013 hPa. That common reference makes sure all aircraft are correctly separated when flying and ensures common language is used between all the different aircraft and between the aircraft and the Air Traffic Controls. Altitudes given in Flight Level (e.g. FL350) refer to the 1013 hPa isobar.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
Of course, the principle shown in figure B8 is theory. When an aircraft flies over long ranges, the weather conditions change continuously. In particular, at a given geometric height, pressure varies. Or the other way around, for a particular pressure, the geometric height will for sure vary.
Therefore, when flying along an isobaric line,
.
In LP zones, the aircraft actually descends relative to lifting air in order to maintain pressure altitude. Hence aircraft performance is slightly better than reality (since Mach number slightly increases).
.
In HP zones, the aircraft actually climbs relative to lifting air in order to maintain the pressure altitude. Thus, the aircraft performance is slightly worse than reality (since Mach number slightly decreases).
The aircraft vertical velocity can be estimated from the wind and pressure forecast maps at a given FL and on a given sector. On this type of maps (see Figure B9), Isobaric or iso-altitude lines are indicated. As a reminder, 1 hPa near the ground is equivalent to 28 feet while 1 hPa at FL380 is equivalent to 100 feet.
Figure B9 - Isobars FL100/700 hPa - iso-Altitudes, temperatures and winds
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
Weather offices can provide isobars at different altitudes, indicated in Flight Levels (FL): FL50/850 hPa, FL100/700 hPa, FL180/500 hPa, FL250/300 hPa, FL340/250 hPa, FL390/200 hPa.
Thus, as a result of the isobaric surface slope, the aircraft may be flying uphill or downhill depending on the pressure field. In performance demonstration flight test, isobars are usually followed to minimize drift angle. In airline revenue service, this is not feasible since airways cut across the isobars.
The isobaric slope can be related to the drift angle as illustrated on figure B10.
Figure B10 - Isobaric slope and drift angle 中國航空網 www.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：getting to grips with aircraft performance monitoring(14)
