#338
Misd-agin;
What sort of radar management settings are you talking about when you say the cells "are all but invisible on radar"?
At the recommended settings many cells are "all but invisible on radar". For that reason alone most guys don't use the recommended settings and instead set the radar at it's most sensitive settings and work backwards, towards less sensitive settings, from there.
Precisely. Works every time - "manual" calibration at the highest sensitivity then back off to the setting before "auto".
Antenna tilt is crucial to building the whole picture of what's up ahead - of this, more in a moment.
For the general information of the majority here who don't fly or don't fly heavy transports or the route under discussion it is important to understand some basics about radar, specifically the A330's.
To put it more succinctly and clearly, the following is offered to SLFs and other interested individuals who do not desire to second-guess any accident crews' decisions in advance of the facts or posit theories based upon guesswork but who instead seriously wish to learn, and who may wonder what radar techniques are routinely used and what radar can and cannot do. The following is standard equipment in an airline pilot's toolkit and is, or should be, unremarkable in terms of knowledge.
I am referencing ONLY the A330 radar here. The knowledge/statements are for information only and are not definitive. That's what the AOM and your company's MANOPs are for. A lot of this comes from Dave Gwinn's "How Radar Works", as well as just plain experience.
First, it should be understood that reading the radar signal is somewhat of a practised art and, like reading DFDRs and QARS requires some experience before interpreting the signal can be done well.
The proper use of digital, flat-plate radar requires an understanding of the nature of the signal and practical experience to interpret the returns with accuracy in order to plan a route through a line of thunderstorms or how much to avoid a thunderstorm by. The reasons for this will become clear.
Radar only senses (returns signals from) water. Radar does not detect "clouds" per se because some clouds, especially high ones do not contain moisture but only ice crystals which reflect radar signals very poorly.
Radar does not detect snow effectively enough for use/avoidance. I believe radar detects super-cooled liquid water but I cannot reference same in anything I've read.
The techniques for use of radar when pointing the antenna above the freezing level are different than when pointing below the freezing level.
Radar cannot detect CAT, (clear air turbulence - even if there is a 'doppler' mode on the installation - I've never seen that mode work successfully - anyone?). Radar may detect ice crystals but very poorly. Radar is not used to detect other airplanes or birds.
The A330's radar return is presented digitally in 3 colors - green, amber and red. I cannot quote the density of moisture which returns each color. Crews are advised to avoid anything in amber.
Radar returns can seem to exaggerate moisture content close in, (40nm scale and smaller). What can look "serious" on the 20nm scale can reduce or disappear on the 80nm or 160nm scale.
Avoidance:
A rule of thumb, (and it is only that), is to avoid strong (red) returns by at least 10nm found below the freezing level and 20nm above.
Signal attenuation:
Thunderstorms being seen on the radar can block thunderstorms behind those returning the radar signal until one is either past, over (not bloody often!) a thunderstorm, or the thunderstorm is dying and gets out of the way of the next ts's in line. Picking one's way in a line of ts's is difficult work in a rapidly changing environment.
Radar antenna tilt:
The A330's radar is IRS-stabilized so regardless of the pitch attitude, the radar is always "level" with the horizon, (in quotes because, to be accurate, there is more than one definition of 'horizon'. But practically speaking, at zero degrees tilt, the radar is pointed to the horizon we normally see regardless of pitch attitude.
The A330's radar beam is about 2.84deg wide, (3deg for all practical purposes - same with the 340 and 320 I believe - check with the AOM). That means that antenna tilt must be used frequently to scan up and down for moisture which indicates ts activity.
The 1-in-60 rule can be used to guage a rough altitude of the signal being returned by moisture, which, depending upon it's shape, (hooked, curled, tightly-banded with amber/red etc), indicates convective activity.
A quick calculation for a very rough altitude at which the radar's beam center is pointing is given by the formula,
antenna tilt x distance (on the radar screen or, in the case of the A330, the ND scale) x 100. For example, at 1deg down-tilt, the center of the radar beam at 80nm in front of the aircraft is 8000ft below the "horizon", (practically speaking, "below the aircraft"), and at 160nm ahead, the beam-center is 16,000t below the aircraft.
Because the beam is about 3deg wide, the returns are, effectively, 3 x the distance x 100. At 80nm, a 3deg beam is 24,000ft wide.
By scanning "up", one can just catch the highest altitude at which water (not water vapour) is present, knowing that convective activity can go higher. Practically speaking again, a tilt of 3deg "UP" puts the bottom of the radar beam on the horizon. A zero-tilt means the radar is scanning 1.5deg above the horizon and 1.5deg below. One can easily work out the heights scanned at varying distances ahead of the aircraft from there.
Eighty nautical miles is okay for avoidance but 160nm is preferable as it provides a smaller detour angle and also permits one to see thunderstorms attenuated by the ones in front, much earlier.
By taking frequent "slices" up and down and using this knowledge, one can begin to build a slightly better picture of the convective activity ahead, that contains moisture.
All this said, every airline pilot knows that it is not good strategy to attempt to outclimb (overfly) a thunderstorm. We know that the height of convective activity can exceed the radar returns sometimes by a substantial margin and that when such activity is stopped in it's climb by the tropopause, if the thunderstorm is severe enough the resulting overhangs can contain hail thrown from the center of the building storm, so avoiding the overhangs by a wide margin is done.
We know that strong to very violent turbulence is indicated by curling red returns, (vortexes, essentially), as do hooks and very tight-grades (narrow bands of amber-against-red signals).
Every airline pilot knows that radar is used for weather avoidance, not weather penetration.
A moon-lit night provides excellent viewing of convective activity. Often one can, along with the distant tell-tale lightning, see the actual storms and, using radar, pick one's way between the build-ups. For the info of those that don't fly, it's done every day, thousands of times, with unremarkable success.
These techniques are not definitive. Within a reasonably narrow band defined by the radar installation itself, varying approaches to using and reading radar are used.
My only speculation at this point would be, whatever else the operating AF crew was doing as part of their SOPs for such weather, they were almost certainly looking out the windscreen while using the radar as described. As to the rest, I cannot fathom how, and why, anyone would speculate here or anywhere as to what happened, under the present state of knowledge.
PJ2