The only valuable approach today is to compute this slope from inertial information. This then would include all possible isobaric slope effects (pressure or temperature, geostrophic winds) without having to distinguish between those.
3.4.2.3. Winds and Pressure zones Let us start with basic reminders on winds and pressure zones.
3.4.2.3.1. Wind
At high altitudes, the wind direction follows isobaric lines, while at low altitudes, the wind direction cuts through isobaric lines.
As illustrated on figure B15bis, when crossing over isobaric lines, and when in the North hemisphere (the contrary for South hemisphere), -if left hand wind, the aircraft flies from a high pressure zone to a low pressure
zone -if right hand wind, the aircraft flies from a low pressure zone to a high pressure zone
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
Figure B15 bis - Wind / Pressure zones relationship
3.4.2.3.2. Pressure versus wind relationship
Pressure variations are linked to the wind velocity. Indeed,
.
Low wind velocity corresponds to slow pressure variations
.
High wind velocity corresponds to quick pressure variations
Thus, at a given flight level or pressure altitude, successive isobaric lines are distant with weak wind, close with strong wind.
As a result, the wind force is linked to the pressure distribution, and as of a consequence, it has an impact on the actual aircraft profile.
3.4.2.4. Low and high pressure zones The low pressure (LP) zones are small and scattered. The isobaric lines are
concentrated and close to circles. In these LP zones, the air is unstable and climbs strongly. Some turbulence may be encountered.
The high pressure (HP) zones are wide. Isobaric lines are distant and have awkward shapes. In these HP zones, the air is stable and gently descents.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
BACKGROUND
Figure B16 - Example of HP and LP zones
In practice, the air mass vertical velocity cannot be measured on board the aircraft. The aircraft trim is modified to maintain pressure altitude.
In Europe, statistical air vertical velocities encountered are centimetric (from 0.01m/s to 0.1 m/s). Worldwide, the mean value of vertical winds encountered is
0.6 m/second.
Most of the monitoring procedures probably do induce a unfavorable bias in cruise performance measurements because the crew usually concentrates on calm atmospheres. As explained above, extremely calm atmospheres necessarily correspond to sinking zones since these tend to increase stability. The problem is therefore to estimate the bias that can be attributed to vertical winds. As a preliminary study, the specific range deviation generated by an air mass vertical velocity was established on an A320 aircraft model and was equal to a DSR of 1% for 0.17 m/second.
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