Why will windshear inop illuminate on the annunciator panel after landing or before starting taxi in a McDonnell Douglas MD-83?


The WINDSHEAR INOP light tells the pilots that the Predictive Windshear System cannot work:

System Annunciator Lights


An amber WINDSHEAR INOP annunciator is installed on the annunciator panel. This light illuminates any time the windshear systems detects a fault which renders the system inoperative.

(MD-80 FCOM Sec. 16 - Instrumentation / Navigation / Communication - Predictive Windshear System)

In order for this system to function, the radar system needs to be active, which is a hazard to the ground crew:

RDR-4B Radar System Theory

Like all modern radar systems, the RDR-4B operates by emitting short intense pulses of microwave energy which are reflected by objects having reflective characteristics within the range of the system. The reflected signals are processed to produce visual displays that are representative of the size, intensity, bearing and distance of the targets. The RDR-4B incorporates an advanced microprocessor design which utilizes the Doppler principle to identify areas of moderate and higher turbulence as well as low level windshear (microburst) activity. The RDR-4B system is designed to operate in the windshear mode automatically any time the aircraft is below 1500’ AGL, at least one engine is running, and the transponder is not in OFF or STBY, REGARDLESS OF RADAR MODE SELECTED. The system also operates in the windshear mode below 1500’ AGL using an alternate scan technique if the radar is operating in any mode (WX / TURB, WX, MAP). The windshear mode is activated at 2300’ AGL. However, no alerts or displays are annunciated above 1500’ AGL.

WARNING: Failure to return the transponder to STBY after landing or selecting any mode except STBY prior to leaving the ramp area allows the radar to operate in the windshear mode creating a radiation hazard to personnel on the ground. The RDR-4B radiation hazard area extends 13.4 feet from the radar antenna in a 120 degree arc left and right of the aircraft centerline. This hazard exists even if the radar mode is selected OFF or TEST.

(MD-80 FCOM Sec. 16 - Instrumentation / Navigation / Communication - Predictive Windshear System)

After landing, the pilots will switch the transponder to STBY in order to deactivate the radar system before reaching the apron (where ground crews are working). This will disable the Predictive Windshear System and causes the WINDSHEAR INOP light to illuminate.

  • 3
    $\begingroup$ Is it just my unfamiliarity, or is that not the safest way of implementing emission control? Having the radar continue to radiate unless the crew flips the transponder switch sounds a bit unsafe compared to, e.g., having the weight-on-wheels signal inhibit the radar. $\endgroup$ Dec 10 '19 at 11:25
  • 2
    $\begingroup$ @AEheresupportsMonica You don't always want to inhibit the radar on the ground since you would otherwise not be warned of a windshear during the takeoff roll. But I agree, this is probably not the best way of implementing it... $\endgroup$
    – Bianfable
    Dec 10 '19 at 11:26
  • 1
    $\begingroup$ Yeah, that's fair. But it could be done by arming the system such as, with WoW off, it going on and remaining on for 5 seconds, would shut the radar down. Makes me wonder how this is handled in more modern aircraft. $\endgroup$ Dec 10 '19 at 11:31

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