The Use, and Misuse of Weather Radar – Part 4

In the final article of this series we’ll look at another one of the questions that we address on a regular basis. Will it damage the radar to turn it off right after landing? Many people have been told that it will damage the antenna as you bounce around while taxiing or even going through rough air in flight if the radar is off. If you look on the different pilot forums you will see differing opinions. You might even check the airline pilot forums because surely those guys would know – right? And there you’ll find very definite answers - yesses, noes, and maybes. So, what gives?

From a design perspective ARINC 708A defines the maximum antenna pointing error and the system error budget. The total beam pointing error is expressed as a root sum of squares (RSS) and the tolerances are allocated as follows:

Radome                                 0.25 deg

Attitude Accuracy             0.25 deg

Tilt Resolution                   0.25 deg

Tilt Accuracy                      0.25 deg

Antenna Mount                  0.1 deg

Antenna Accuracy           0.25 deg

The Total Beam Pointing Error (RSS) derived from these tolerances is 0.57 degrees.

As a radar manufacturer we are responsible for meeting the following tolerances: Tilt Resolution <= 0.25deg, Tilt Accuracy <= 0.25 deg and Antenna Accuracy <= 0.25 deg. The maximum RSS of these tolerances is 0.43 deg. Those tolerances which are beyond our control, Radome, Attitude Accuracy and Antenna Mount have an RSS allowance of 0.37 deg.

Pointing accuracy is one important requirement but there are other practical requirements. We must be able to sweep the antenna back and forth at the desired rate, turn it around swiftly and smoothly at the end of each sweep, and be able to move the antenna under extremely cold conditions. For these we need adequate torque.

And this is why you get different answers to the same question. If everyone met the requirements with the same design we wouldn’t have high-wing aircraft, low-wing aircraft and mid-wing aircraft. We’re always trying to design the better mouse trap.

There are a couple of methods commonly used in airborne radar antenna design. One method uses AC synchronous motors that spin at high RPM to generate torque. But clutches and gears are needed to reduce that high RPM down to the much lower RPM that the antenna scans at, and to make it turn around at the end of the sweep. These types of antennas are more commonly used on air transport aircraft. Because the clutches are not engaged when power is off, they can bounce around when taxiing if the radar in unpowered.

The other main antenna design uses stepper motors. Stepper motors produce the required torque, but the steps are large and don’t provide adequate resolution. So, gears are required to provide the necessary resolution. On these antenna drives it takes considerable effort to move the antenna when power is off, and it will not bounce around.

For the purpose of this article, we will only consider the Honeywell Primus 880/660/440, BendixKing RDR-2000/2060/2100 and the Honeywell RDR-4000 and RDR-7000 systems. All of the radars just listed “except the RDR-4000” use stepper motors and it is safe to turn power off during taxi or any other phase of flight.

The same is true for the RDR-4000 but for a different reason. The RDR-4000 uses direct drive DC brushless motors. There are no brushes, bearings or contacting components. Instead, it uses only the larger gimbal bearings. There are no gears, clutches or mechanical backlash adjustments making it extremely quiet and reliable. When the motors aren’t powered they move freely but the antenna mount has a counterweight that is equal and opposite to the weight of the flat plate antenna so the flat plate floats weightlessly making it safe to power off. 

The next time they have the radome open on your aircraft for maintenance try the following. Make sure the power is off and go up and grab the antenna flat plate firmly on each side and move it a little bit. If you feel resistance and hear the gears moving you have stepper motors and a gear system.  You’ll quickly realize it takes a good amount of force to move the antenna and it isn’t going to bounce around. This is the primary design type on business aircraft and helicopters.

Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached via email at stephen.hammack@Honeywell.com.