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The technique for operating the weather radar effectively utilises a combination of range and beam
depression or elevation referred to as tilt. Guidance on the range setting that should be set on the ND
for each pilot for avoiding thunderstorms recommends that the PF selects his ND to 80 nm and the
PNF to 160 nm. The ND should be set to a range of 40 nm when in 'Turbulence Mode' and the
antenna tilted to avoid ground returns. The importance of readjusting the tilt frequently in order to
monitor storm development and to get the best cell echo is emphasised. Failure to tilt the antenna
down periodically may cause a target to disappear.
As the 0° tilt angle is slaved to the horizon, a formula is provided for calculating the vertical distance
between the top of the weather cell and the aircraft flight level. The tilt angle element of the formula
is based on adjusting the tilt until the echo begins to disappear and then noting the tilt angle.
The company provide operating procedures, titled 'Encountering Adverse and Potentially Hazardous
Atmospheric Conditions' covering the situation where thunderstorm activity is detected either visually
or by using the weather radar. The information includes 'Techniques for Flying Through Areas of
Thunderstorm Activity' and the advice given is that above 30,000 feet, 'avoid all echoes by 20 miles'.
Advice also states that 'the pilot should not attempt to penetrate a cell or clear its top by less than
5,000 feet vertically, because otherwise the aircraft may encounter severe turbulence. If the top of the
cell is at or above 25,000 feet, overflying should be avoided due to the possibility of encountering
turbulence stronger than expected'.
A formula for calculating the vertical distance between the top of the weather cell detected on the
radar and the aircraft flight level is provided as:
Vertical distance (feet) = range of cell (nm) x radar depression angle (°) x 100.
Weather radar training
Crew training in the use of the weather radar is carried out during the aircraft type conversion course
and initial line training. Procedures, set out in the 'supplementary techniques' for the use of the
weather radar mentioned previously, and training videos, covering the use of the weather radar and
adverse weather operations, are also provided by the operator.
Airbus A321-231, G-MIDJ
17
Recent developments
The latest weather radar equipment, called a 'multi-scan system', incorporates an antenna that not only
automatically scans left to right but also up and down. This system also incorporates automatic
ground clutter suppression allowing only weather to be displayed. By scanning the whole atmosphere
precipitation at the bottom of the thunderstorm cell, that is normally hidden within ground clutter, can
be detected. Thus thunderstorms can be displayed more clearly and sooner. The present in-service
systems' detection level is poorer in that when set to scan only the top of a thunderstorm cell they will
only be targeting levels of the atmosphere where only dry ice is present. This newer equipment thus
encourages aircraft track adjustments around thunderstorm cells that may contain areas of severe
turbulence present at levels undetected by the present radar equipment.
Flight Management Guidance System (FMGS)
Having checked that the aircraft systems were in a safe condition, the crew used the FMGS to
determine whether the aircraft had sufficient fuel remaining to continue the flight to Manchester.
They decided that an en-route diversion was not required and by remaining on their planned route the
fuel required on the flight plan at each way-point could easily be checked against the fuel available.
The FMGS 'fuel page' provided a calculation of the fuel required to continue to Manchester, at the
lower level of FL230, as well as any extra fuel that was available. The extra fuel available just before
the aircraft entered the hail had been 900 kg. The fuel calculations made by the FMGS, however, are
based on aircraft performance data stored in the computer. This data (also available to the crew in
hard copy), calculated on the assumption that the aircraft is aerodynamically undamaged, and not on
fuel actually being consumed, is used to establish the fuel required. The extra fuel figure is
continually re-calculated by the FMGS by subtracting the fuel required from the fuel available.
The radome and aerodynamically significant airframe surfaces of G-MIDJ were damaged by hail and
the total drag coefficient of the airframe, and thus its performance were, to some unknown extent,
affected. The FMGS fuel calculations did not take account of this change and thus the fuel required to
destination, displayed to the crew, was somewhat unrepresentative. In the event the resultant increase
　
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