View Full Version : Weather Radar Usage Boeing question

26th Jan 2010, 08:57
Looking for help from the experts out there.....with regard in particular B757/767 radar's although the response may be of interest to other public transport drivers.

Under JAR and FAA ops it is mandatory for public transport aircraft to be FITTED with Weather Radar.

MEL's also include restrictions on aircraft flying with an INOP Radar.

I am currently flying for an unamed operator flying B757, B767 and there is a general 'acceptance' to fly with the radar turned off.

I am sure this is wrong but cannot place my finger on the ink that says so.......
My argument would be that it is a 'tool' that was introduced and made madatory on public a/c to enhance safety. With restrictions on a/c flying mentioned in the MEL I would argue that it should be selected ON for flight......

So.....can you help and/or provide answers to the questions below.

1. What actually happens when the Radar switch is first turned on ?

2. Is there any requirement/mandate when and where it must be used ?

3. How does it interact with the GPWS system ?

4. How is the GPWS system turned on ?


Capt Fathom
26th Jan 2010, 10:16
I'd have thought the 'how it works' would have been covered by the ground school.

SOP's and common sense would dictate when you use it!

26th Jan 2010, 11:06
To try and answer this query:
1) With modern radars, the modulator within the TxRx will start to fire within seconds of being switched on (unlike to old 'RCA' type which took an eternity).
2) Darn good idea to use ALL of the time while operating the A/C.
3) Assuming you have EGPWS fitted, PREDICTIVE Windshear Alert is generated by the radar. (Again, assuming that this is modern Bendix/Honywell kit). REACTIVE comes from the EGPWC.
4) GPS/EGPWS is running full time; there 'aint an On/Off switch. (Just Gear/Flap O/Ride switches in many installations).

26th Jan 2010, 11:08
Oops, I of couse meant GPWS, not GPS...:*

26th Jan 2010, 11:58
"general acceptance to fly with it off."

Can't understand that one. It's free; it doesn't cost the pilot a shilling for the meter, and could save you embarassment of explaining how some pax or C/A's came to be dancing on the ceiling. In severe clear skies I can understand it being off, but that is an airmanship decision. If you can't see where you are going to be in 5 mins then it should be on.

26th Jan 2010, 11:59

About weather radar, the MEL for the airplanes I operate (A3xx) states the following : may be inoperative when weather reports indicate that thunderstorms or other potentially hazardous weather conditions, which can be detected by a weather radar, do not exist enroute(etc..)

2. Is there any requirement/mandate when and where it must be used ?

Yes, see above. But before being a requirement it is a matter of common sense... I mean you use wxr to avoid bad weather... during daylight in cavok conditions you can leave it turned off.
Another great issue is knowing how to use wxr in a correct way: aircraft manufacturers often write about correct operations of airborne wxr in their training manuals and AOM and it is very important to know how that system is working in order to use it in the best way. Messing around with TS and Cbs can put you in very bad situations and must be avoided at all times. Wxr is a tool designed to avoid Cbs in a correct way, and for that it is important to review sometimes how a Cb works, and where you can fly and where you must not.
Anyway just my personal opinion:ok:


26th Jan 2010, 20:18
1. What actually happens when the Radar switch is first turned on ?
2. Is there any requirement/mandate when and where it must be used ?
3. How does it interact with the GPWS system ?
4. How is the GPWS system turned on ?

First, let's look at the Collins or Bendix/Honeywell weather radar that has been equipped on the 75/76 since their beginning 1982. The devices are solid state, so self-test and antenna synchronization takes only a few seconds. The transceiver operates to detect rainfall and precipitation related turbulence only when at least one pilot has selected WXR display.

2. If you can dispatch radar inop when no TS are predicted, then having it energized when no TS visible makes no sense. Flight at night is another matter.

3. Weather radar is standalone, and not connected to GPWS.

4. GPWS is on all the time. EGPWS, or the generic TAWS, Terrain Alert and Warning System is on all the time, too.

Predictive Windshear Weather Radar, or more correctly, Forward Looking Windshear Weather Radar, is independent from GPWS or TAWS, and became available in 1995, when Windshear detection and warning was mandated in the US. It is energized when there is oil pressure to one engine and Transponder On, meaning the plane has left the gate, scanning the sky for windshear events, although the display may be off. It operates automatically to about 2300 feet on departure, sharing the standard rainfall detection task if the pilot has engaged his weather radar display.

Likewise on the approach, the FLWS turns on at 2300 feet, providing alerts and warnings as needed, regardless whether the pilot has turned on his display of rainfall and turbulence detection.


27th Jan 2010, 14:16
Weather Radar have to be ON in certain region (military conficts close by) so that your plane will be identified as civilian...
Now leaving it on all the time is NOGOODLAA, because the antenna will deteriorate and side lobes will be an issue.
So what is the best? If your flight is well above the tropopause then switch it off, otherwise look at your significant weather chart to see where it is needed due to weather, and mark it on your Flight plan paper copy, (so that you will not forget it) and buzz off.
As far as I am concerned I leave it ON. :ok:

27th Jan 2010, 15:23
Thanks for all who have replied thus far........

I tried asking the question in a way not to prempt or lead an answer......

Let me further say....and reply without directing to any one individual.

Most people are aware of what is taught in ground school and how things work ina general way. Just need to look at Mr Boeing Manuals to see how much these days has been removed and no longer covered.

As far as SOP's and common sense go, I have seen examples of where both are somewhat lacking.....

I am aware that MEL;s have placed restrictions on the type of flights made and the despatch of a INOP radar. BUT......
the MEL mandates when despatch may be made NOT that it has to be used !

I am aware that there is a notam affecting flights into 'sensitive' area's whereby you are REQUIRED to use it......or risk being shot down.

Looking at EGPWS I know that when you turn the thing on (RADAR) it automatically sweeps and scans for Windshear, which is great as you line up just before takeoff........

BUT if GPWS is 'ON' all the time then when is it first powered'......and how or rather what activates it.........if as mentioned above when oil pressure is indicating that and engine is up and running and 'left the gate' then what is the actual process involved........

ALSO if EGPWS ....windshear detection is first scanned when the RADAR is first switched on then how does the system actually work......
Vague recall about mode 7 of EGPWS......(currently downroute & no access to loads of books..etc etc)

Thus I come back to the original questions.........

Is it written in black and white where I can point my finger and say........

I knew it would potentially open up a can.....

By the way I do fly with it on and utilise tilt/range to interrogate wx ahead but I'll be darned if I can find the answers anywhere thus far....!

27th Jan 2010, 15:53
Go back to the root, use the Pilots Guide published by the company who made your Radar. :ok:

27th Jan 2010, 16:10
I must not have been clear in my prior post, BGA. . GPWS, Ground Proximity Warning System and EGPWS, Enhanced GPWS are entirely unrelated to FLW, Forward Looking Windshear, except for priority of alerts and warnings.

The startup logic for FLW when leaving the gate: oil pressure on either engine, transponder ON and less than 2300' radio altitude. It begins with a self test, and then scans for microbursts. Alerts and warnings are inhibited above 100 knots and below 50' radio altitude. Once above 2300' radalt, it shuts off.

Crossing 2300' radio altitude on descent. turns it on again, of course.

I have not heard of WXR being used as IFF. Sounds bogus to me. The beam is only in the forward hemisphere. and not necessarily aimed at ground installations. I have seen spokes on the display that point to military facilities, however, due to ground based radars on the same X-band frequency.


27th Jan 2010, 16:22
It is quicker to state that it is Bogus, then to do so research on it,:} it goes back to the period when an A300 Airbus IranAir was shoot down in the Persian Gulf, Civilian weather radar have a different signature from military tracking Radar.

27th Jan 2010, 18:27
It's somewhat technical, but the Iran Air shootdown occurred when the USN ship's IFF confused the 6400 feet Mode C altitude reply from the A300 with a discreet Mode A code often used by the Iran AF.

Meanwhile, airliner weather radars have evolved from 60KW peak power to 150w peak power in a scanning pencil beam. That's hardly comforting to think a military would depend on that to distinguish a transiting airliner from an enemy intruder.

Besides, the KC-10A tanker and other USAF planes use commercial WX radar..

27th Jan 2010, 18:53
Greetings, I never stated that they rely only on that to identify.....

28th Jan 2010, 06:17
It is energized when there is oil pressure to one engine, meaning the plane has left the gate,

Sounds a bit dodgy... What if the aircraft had to start an engine at the gate due to maintenance reasons.

Our 747-400 books say that takeoff thrust achieved on #2 or #3 engine will activate predictive windshear. This is more likely to be an rpm value, rather than oil pressure. Haven't checked our 767 manuals yet.


28th Jan 2010, 09:53
As Windshear was on the top ten list, authorities have asked airframers to wire the system in such way that predictive windshear will be available at takeoff, even if WXR Radar is switched off, so all sorts all mods had to be done, from oil pressure during engine start (I believe B737 is the case) to takeoff thrust and 80kts.
the configuration is more related to what could have been done to implement the requirement, this is why you will hear different stories.:ok:

28th Jan 2010, 10:43

I am aware that MEL;s have placed restrictions on the type of flights made and the despatch of a INOP radar. BUT......
the MEL mandates when despatch may be made NOT that it has to be used !

If I understand your point, you are talking about general company policies on use of weather radar, right? You might have radar on required for takeoff everytime as per SOP, but after that having it on or off could be left upon crew discretion if there is no significant weather. What is mandatory is to use radar when there is weather ahead and making a good assessment of the situation.

Bye !

28th Jan 2010, 11:06
Modern weather radar units still require the use of some mechanical parts. These include gyros; jack-screws; and various bearings, seals and lubricants.

In simple terms, it's cold outside. Somethings as cold as -60. I doesn't take a rocket inventor to understand what happens when mechanical objects are subjected to that level of cold; then snapped on and expected to work without impediment.

Most people shut these things off out of fear of the radiation exposure. However, (and I'll happily provide the reference document) the level of radiation from that modern aircraft radar, is about HALF that of your typical modern Hand-phone.

Personally - like someone else stated earlier - I also believe:

I costs nothing to use,
Using it properly – even in clear conditions – continually reminds you that it IS WORKING, and
There are NO adverse effects to using it.Your company SOP should always prevail, but given the overall - and obvious - lack of knowledge with WX Radar systems, it's no surprise that so many companies simply don't address its regular use in their SOP.


28th Jan 2010, 13:28
OK, I checked a Lamm diagram for FLW, and retrofit installation is one engine running (oil pressure discrete), transponder ON, and less than 2300 feet radio altitude.

You want the system working before taking the runway for takeoff.

Alert Inhibits:
*Above 100 knots and Below 50 feet on takeoff
*Above 1200 feet radio altitude
*Targets Beyond Touchdown Zone on Approach

I'll edit my prior post. Sorry for the confusion.


Captain Smithy
28th Jan 2010, 13:30
As a radar engineer I often read radar-related threads around here and observe the comments with much interest. I'm not a Boeing driver (light singles only) so I cannot comment on SOPs for radar use, however there are a few points I'd like to comment on.

In simple terms, it's cold outside. Somethings as cold as -60. I doesn't take a rocket inventor to understand what happens when mechanical objects are subjected to that level of cold; then snapped on and expected to work without impediment.

A point well mentioned, as the effects of cold are also present on the electronic components. Thermal shock damages components and is the main cause of failure. Cold-soaking a radar at -40C for a period of time and then powering it on (therefore heating it up) will subject the components to significant thermal stress and will very quickly cause failure. If the unit is powered-on on the ground, e.g. when lining up on the runway, the circuitry will already be warmed up and therefore the effect of extreme cold at altitude will not stress the circuitry as much. Thermal stability is an important consideration with most electronic systems, applying no less to radar.

Most people shut these things off out of fear of the radiation exposure. However, (and I'll happily provide the reference document) the level of radiation from that modern aircraft radar, is about HALF that of your typical modern Hand-phone.

Not sure about the comparison with the mobile phone power but sitting behind a transmitting radar will NOT have any affect on you. The microwave energy is radiating away from you, and you are sitting behind it. Nothing to worry about. A lot of people are scared of radar and RF tarnsmissions and their affects on health. Much interesting work has been published on the subject and is readily available via a search engine but the fact is it will have no affect on you, unless you are daft enough to stand in front of a transmitting antenna, in which case you will feel a bit warm and have your balls sizzled. :suspect:

My take on it - if it's there, use it. :)


28th Jan 2010, 14:04
[quote]Cold-soaking a radar at -40C for a period of time and then powering it on (therefore heating it up) will subject the components to significant thermal stress and will very quickly cause failure. /QUOTE]

Astute avionics manufacturers have always stressed incoming components at . . . . -40C . ., to knock out the ones that will fail prematurely. (That's part of why avionics are so expensive.) Wx radar antennas are designed to operate either occasionally or continuously at altitude, whether started warm or cold. Typically they average more than 10,000 hours flight time between unscheduled removals.

Some airlines automatically remove the antennas for lubrication and replacement of components that wear out at less than 10,000 hours, so their MTBUR, Mean Time Between Unscheduled Removals is much higher. Running the radar continuously will wear out the antenna pointing mechanism sooner, guaranteed.

The Wx radar transceiver has no moving parts, and is inside the pressure vessel in transport category aircraft, so not subject to extreme temperatures, of course.

How much does it cost to operate the Wx radar? Probably on the order of $1 per flight hour.

Some pilots find the radar return on the Nav Display to be a nuisance, and turn the intensity down to 0. It would be better to just turn it off.

Historically, UAL has been the only carrier to provide adequate Wx radar training to pilots.


28th Jan 2010, 14:10
...As a radar engineer... I think this constitutes an expert on the subject!

Furthermore, previously two Aircraft Engineers from AirXXs provided the same info as Smithy.

28th Jan 2010, 15:08
The rest of the world seems to use the name "engineer" for jobs which in the US are called mechanics or technicians.

How do you all distinguish between:
the guy who conceptualizes a new Wx radar system..
the guy who designs the innards of the Wx radar components..
the guy who lays out the wiring and mechanical installation of the weather radar system..
the guy who provides technical interface between the manufacturer and the user..
the guy who troubleshoots and replaces a wx radar LRU, Line Replaceable Unit..
the guy who repairs the LRU..


28th Jan 2010, 15:23
How do you all distinguish between:

the guy who conceptualizes a new Wx radar system = Designer

the guy who designs the innards of the Wx radar components = Engineer

the guy who lays out the wiring and mechanical installation of the weather radar system = Assembler

the guy who provides technical interface between the manufacturer and the user = Support Representative

the guy who troubleshoots and replaces a wx radar LRU, Line Replaceable Unit = Aircraft Line Maintenance

the guy who repairs the LRU = Avionics Technician

That was easy. Next question?

28th Jan 2010, 15:47
Thanks, Pappy. However, for most of them you provided the job description, not job title. For example, "Aircraft Line Maintenance" seems to be called Engineer in much of the English speaking world.

The guy making drawings, Engineering Orders and diagrams in the Engineering Dept. of an airline is what you call an assembler? I guess my English didn't translate. This is who I meant when I wrote, "Lays out." Sorry.


28th Jan 2010, 16:36

Our Airbus (A-320) FCOM says the following in the Taxi checks

RADAR (if required) - ON

My understanding is if Airbus wanted the Radar to be ON all the time, they would not have written (If Required)

As mentioned earlier, these radars have a lot of moving parts and all of them have a MTBF. So by switching them Off when not needed is prudent.

So keep the radar ON for Takeoff but when settled in the climb/cruise and with unlimited visibility and with not a cloud in sight, it is quite Ok to switch it off

My two cents

Captain Smithy
28th Jan 2010, 17:37

Indeed you are correct, avionics manufacturers employ an environmental stress testing in order to "weed out" weaker components and bring out other issues e.g. poor soldering. However issues can still appear post-test ater the unit has left the factory floor and is in service.

What kills electronics is thermal cycling - rapid heating up and cooling down. The most common way this can happen is the continuous cycling of power - i.e. switching on and off. Most failures occur at switch-on; at switch-on there are brief spikes of voltage and current which unavoidably over time damage the components, eventually causing failure. Obviously switching on-and-off also causes the components to heat up and then cool down again respectively; even in mil-grade components this causes damage over the long-term.

An interesting point to note is the decreasing availablility of mil-spec components; both suppliers and customers are growing increasingly wary of the high price (and, for suppliers, the low demand and hence small market) of mil-spec components, so increasingly avionics manufacturers are using mass-produced commercially-available components which can be found in anything from home computers to alarm clocks, and hence not spec'd to the more extreme temperature tolerances that mil-spec components have. Beancounters brilliance of course. Another reason why the free-market isn't always the answer to everything, despite what others think ;)

With this in mind, it is easy to see why it is not a very good idea to continuously switch a radar on and off when cruising at FL300+ and the OAT is minus f-ing cold. Doing so on a regular basis over time will stress the radar components and will ensure an early visit to the workshop or a return to the factory. The radar installations I deal with are still in non-air-conditioned/nonpressurised environments (although typically processing equipment tends to be otherwise), although they usually have their own cooling apparatus for operation at higher ambient temperatures.


28th Jan 2010, 18:04
Thanks, Smithy.

Lack of mil-spec components means more incoming inspection, including thermal cycling, required to achieve a mil-spec equivalent.

The small amount of heat generated by the electronics in a really cold Wx radar antenna isn't going to raise temperatures very much.

By definition, an airplane is a group of compromises flying in close formation, and the Wx radar antenna is, too. You have to balance the increase in failure rate of the electrical components from power cycles, vs the additional wear of the mechanical components if operated continuously. If the antenna is the older design with no electronics, then mechanical wear is the only factor.

As I alluded earlier, the Wx radar system is reliable enough that operational considerations take precedence over reliability. Probably most US airlines never turn on the radar except at night or when convective activity is visible. If it were required all the time, there would be no on/off switch on the fright deck. Fully half the Wx radar removals in the US occur in TS season, mostly in August.

Back to the 767: the Wx radar is powered only when a pilot has pushed the WX button on his EFIS control panel for display on his EHSI.


Jaun Huw Nose
28th Jan 2010, 19:02
I'm with Pappy on this. As I understand it in the past when we first got wx radar it used lots of power and several a/c in the circuit at the same time meant LOTS of rf which could interfere with the old ATC radars (Smithy can you confirm/deny?)
When I ask other pilots why the switch it off none has ever been able to answer.
Generally its a very badly taught subject in ground school (if its taught at all!)

its free! do you switch off the ADF just because you aren't using it?

29th Jan 2010, 08:52
I'm with PappyJ!! :}

29th Jan 2010, 10:05

its free! do you switch off the ADF just because you aren't using it?

No, but I think the comparison is not so relevant. The ADF that you mention is a mean of navigation and in modern airplanes you cannot "switch it off" unless you enter no frequency in it, but again it is not off, just not tuned.

When I ask other pilots why the switch it off none has ever been able to answer.

In general you leave something off when it is not required, e.g anti ice, ignition, backup hydraulic pumps (electric or engine driven depends on airplanes), etc..

My opinion is that the is not a single way of thinking about that kind of subjects, what is relevant is the result you get in the end:ok:

Captain Smithy
29th Jan 2010, 10:48

Yes other radar transmissions can appear if the Rx is tuned to the same frequency, e.g. if two aircraft are using, for example, X-band radars that transmit (and hence receive) at the same frequency and let's say that they are facing each other, both aircraft's radars would receive each other's transmission. Can make for some interesting effects on the PPI :)


Cheers, no problem. The components that suffer most tend to be power supply circuitry and any processing circuits, which can get very hot quickly. You are entirely correct in what you say however.


29th Jan 2010, 11:39
Graybeard sir, I’m sure that you meant no offence/offense regarding the term ‘engineer’, but speaking as a UK CAA Licensed Engineer may I make a few points here?
In the 1980s It took me 7 years of study to gain licenses in TEN avionics subjects with the UK CAA. (I was no slouch, and passed every exam first time). The system has been simplified now, but it still requires a great deal of study and dedication. In an essay question, you’d be expected sketch and describe the workings of things like a radar Tx/Rx etc, and even if you passed this exam, you then had an oral examination with a CAA Surveyor who would quickly show you the door if you did not know your stuff. I know it’s only a word, but most technicians and mechanics have never had to go through all of this, and their level of knowledge and responsibility is not in the same league. Oh, and of course you need to stay up to date as far as modern avionics systems are concerned as well, the improvements in avionics systems since I gained my licenses are both fascinating and dramatic. Pappy’s explanation here is a little of an over simplification too I’m afraid Aircraft Line Mtce is an area, not a person, the guys who conceptualise and design the system are both Design Engineers. Guys, please do not pigeon hole people without knowing your pigeons. Moan over. :)
GB, Your point regarding the Tx/Rx itself being in a stable environment is of course correct, but as far as I recall my airline has no policy of routinely removing the scanner assembly for lubrication, and our MTBF is still extremely high. And just like the 767, the '400' and 777 systems are also only swithed on when you hit the button
Returning to the subject of radar system output power, I seem to remember that the old RCA system was about 60Kw peak output power, whereas the modern systems (Bendix etc) are around 100 Watts peak. As long as you are sitting BEHIND either system you are perfectly safe. (Can not recall any instances of ATC radar interference though). A point of interest regarding the older high power systems; if the aircraft in question flew at an extremely high cruise altitude, pressurising the waveguide system up to the scanner was absolutely essential. (You typically would used cabin air at the Tx/Rx end, and the fwd pressure seal would be maintained by a Mylar sheet over the flat-plate scanner). Even a small tear in this sheet would result in severe arc-over within the waveguide, rendering both radar systems useless. (I’ve seen the results, it’s as if someone has taken a welding torch to the inside of the waveguide). When the Bendix system was added as a retrofit to this aircraft, there was never another instance of waveguide arc-over.
I tend to agree with Capt Smithy; you are going to do more long-term harm to any system by regularly cycling the power, aircraft systems in general are far happier when they are left running, but particularly cycling a radar system on and off at cruise altitudes is never a good idea. Your radar system is there to help PROTECT you, why fly with your zipper undone? :rolleyes:

29th Jan 2010, 11:49

I was curious....I'll ask you, since you seem to be switched on regarding
RADAR....in the new RADARs, you are warned to be at least 15 meters from the transmitting antenna. (Airbus says not to turn it on when on the ground, if there are people or things within 15 meters.)

But, regarding the old stuff....like the 60 KW stuff you mentioned....what is the minimum safe distance to be when such a RADAR is transmitting?
I've always thought it was 100 meters....but someone, recently, said it was much less.

Just curious....

Fly safe,


29th Jan 2010, 11:58
Hi PantLoad, I recall that 100 (ish) feet was about the safe minimum. In military high-power radars/ecm systems, the distance is farther. There is a huge amount of output power here, concentrated in a very narrow beam.

29th Jan 2010, 13:24
Thanks for the explanation, M2dude, and please excuse the thread drift, as I was confused in prior posts who was what kind of engineer. I'm truly asking to learn. As you must know, signing for return to service in the US requires a licensed A&P, Aircraft & Powerplant mechanic. Engineers in the US usually have more theoretical education than A&P, but are not licensed. They are the design engineers at the manufacturers and the service engineers at airlines who design and document repairs and modifications. Are only your licensed engineers authorized to release for return to service, or do you have other levels of mechanics and technicians with authorization?

Still, a person calling himself engineer without specifying what type confuses me.

Oh, I'm sure most of the airlines have their radar antenna maintenance "on condition," rather than on "hard time." I knew one FAA Principal Avionics Inspector who required radar antenna overhaul at 9,000 hours, as he insisted, rightly, that mechanical parts wear out. Their MTBUR, Mean Time Between Unscheduled Removals, was much higher than other airlines as a result.

As for too many early radars in the area causing ATC radar problems: highly unlikely, as ATC radar has been S band, around 2 GHz, like your microwave oven, while the early RCA radars were C band, above 5 GHz. More likely, too many transponder replies were overloading ATC. That was one reason for the ALT OFF switch on the transponder control panel.

The RCA radars of the 1960s and 70s, AVQ-10 and AVQ-30 and the Bendix RDR-1E and RDR-1F of the same era - and beyond - used magnetrons of about 55KW peak output. (The RDR- and some later RCA are X-band, 9.3 GHz.) That works out to about 750 watts average (heating) power - hmmm - the same as a full size microwave oven. The actual unsafe distance for personnel is a 15 foot arc in front of the radome. McDouglas used to have those areas painted in red on the concrete on the flight ramp where the new planes were parked.

Sometimes there would be rabbit tracks on the radar display in the days of old, from picking up another plane's radar. That was eliminated with the Arinc 700 generation by varying the pulses so the radar would track only its own.

With the 767 came Arinc 700 avionics, and the first new generation of Wx radars, the Collins WXR-700, and the Bendix RDR-4A. They had solid state transmitters of 60 to 150 watts peak power, for an average heating power of less than one watt. You will have to be close enough to get pranged in the head by the swinging antenna before you are harmed by that power level.

Why do these radar distance cautions still exist? Old beliefs and old airplanes die hard. There are still airliners flying with the old magnetron radars.

Meanwhile, your DMEs and maybe transponder and TCAS are probably transmitting continuously with up to 500 watts peak power. Ever see a precaution about being too close to their antennas?


29th Jan 2010, 14:27
Points well made Graybeard, names are confusing I guess, For a ‘box’ change, a technician (or in actual fact an airframe/engine Licensed Engineer) can issue a CRS. (Provided that no external test equipment is used, and the aircraft in question has a BITE Go/No Go system). As a scanner is technically an antenna only an Avionics Licensed Engineer may issue a CRS for a scanner change. Licensing here in the UK heavily splits into specialised Mechanical/Avionic disciplines, whereas the A&P is more general in it’s scope. (Don’t get me wrong, I know several A&P guys in JFK, a couple of which are among the best engineers I’ve ever known). The Mechanical licensed guy here will have limited avionics knowledge/certification powers.
Yep, the biggest danger from a modern radar is to be clocked in the head by the scanner.
I have to admit, I too have often wondered about the DME, Transponder and lower TCAS antennas radiating all this power downwards, onto baggage loaders, refuelers etc, no warning lables on these, and they’re powered all of the time.
Back to the radar switched off issue; on my previous high-flying aircraft, (cruise was betwwen 50 & 60,000'), we also tinkered with the idea of switching the radar to STBY during taxi. This was because the old AVQ-30 stabilisation was constantly fighting the nodding of the nose during taxi, causing premature failures of the antenna servo system and bearings.
Keep posting GB, you know your stuff.

30th Jan 2010, 15:35
With ADF etc and the likes generally we all have received 'training' in using the instrument, and interpreting its information, either as part of the ATPL syllabus or more likely to pass the IR test.
Also with the likes of other equipment eng-anti ice etc ..there are clear guidelines as to when its use is required and can be found in a variety of sources of manuals.
The main reason in the first place in posting these questions came about from trying to ‘teach’ or rather I should say ‘explain’ what I thought was a very ‘easy’ solution. But how do you start when there are no company SOP’s on its use, NO entry in the FCOM about even turning the thing on at all and a clearly general lack of understanding on how the damn thing works in the first place.
I have other sources to try and obtain a ‘definitive’ answer but as this post shows sometimes there is a lack of training and understanding in professional organisation relating to often held old beliefs.
When my own daughter ever asks a question Daddy Why is ....? to be honest sometimes the question is smarter than the answer but I always try and provide a correct answer rather than ‘Because that’s how it just is’.....
To date I have not yet found a definitive source that I can refer to ....to increase my own understanding of ‘how’ the system works and help others .
Maybe I should have been an engineer instead....lol
My thanks to all so far.......keep it coming......

30th Jan 2010, 22:12
BGA you are so welcome. The beauty of a forum such as this, is that fellow professionals can help each other's understanding of such an amazing subject - Aviation :)
(Digging up some info regarding REACTIVE windshear detection within the EGPWC.. Mode 7.. We obviously are looking for a sudden increase in IAS without a corresponding increase in Longditudinal Acceleration at trigger altitude etc conditions, but there is more to Mode 7 than just this; Watch this space). Although by definition Reactive Windshear detection means that you are already 'in it' (the microburst downdraft spread) it is the very early stages of the windshear, and hopefully applying the correct 'escape procedure' gives the pilot a fighting chance to get out of it.

1st Feb 2010, 00:29
If I were a dreamer, I would look forward to mandatory thorough Wx radar training for all airline pilots as one of the results of the loss of AF-447. I would also expect the requirement for workload reducing multi-scan radar, if not by regulation, then by pilot association bargaining.


1st Feb 2010, 00:43
With all due respect to the real Engineers out there I do think the term has been a little corrupted and overused.

In the UK on a layover once I discovered my toilet was blocked, after calling the front desk they sent their (you guessed it) 'Engineer' with the requisite plunger.

He did do a good job though..

Jaun Huw Nose
1st Feb 2010, 09:48
Seconded Greybeard,maybe we should put it to the BALPA tech people to push the CAA?

1st Feb 2010, 10:22
I do think the term has been a little corrupted and overused.

Indeed, in Australia the only State which provides some control over the term (in this case for PEs) is Queensland via the Board of Professional Engineers.

Otherwise, generally the term is used for whatever you want it to represent ... I believe that the problem is quite widespread.

1st Feb 2010, 11:24
Multi-scan radar :ok:

I love this thing!

1st Feb 2010, 12:10
Your unblocker was probably a mechanic or a technician, not a licensed engineer, but he COULD have been.

I seriously doubt it! At a hotel! I believe the term would be "Janitor" or maybe "Plumber"

1st Feb 2010, 12:12
Hugh amount of operating info here


1st Feb 2010, 12:37
Very good info, the WXR-2100 is a nice piece of kit. :D

1st Feb 2010, 16:12
This picture was taken by the FO on a flight in the Far East, it is a typhon and they were entering it :eek:
two raisons, the first one they probably did not have a look at the weather chart, the second one both nd for Radar display were fully dimed, I am sure they will not forget again, this was a close call.

1st Feb 2010, 16:59
I seriously doubt it! At a hotel! I believe the term would be "Janitor" or maybe "Plumber"

Oops. Sorry PappyJ, I'll RTFQ next time. I'm off to unblock my toilet.:}:\

2nd Feb 2010, 07:38
In the UK on a layover once I discovered my toilet was blocked, after calling the front desk they sent their (you guessed it) 'Engineer' with the requisite plunger.

Maintenance Engineers are multi-talented ;) Or rather, the airline, in the name of efficiency, have highly educated people cleaning ovens and trash compactors, washing windscreens, greasing nipples :cool:, removing chewing gum from carpet, etc.

5th Feb 2010, 09:47
Well ladies and gents, thank you for the posts......I have made some further ground and post the following. It is by no means definitive and is a hopefully brief overview with some interesting points....

I knew I had covered some of the stuff but it has been good to refresh some of those old brain cells....

It has gone some way to answering my questions.. I have produced it below and hope that it is of help to some......


The GPWS provides alerts based on radio altitude and combinations of barometric altitude, airspeed, glide slope deviation, and airplane configuration and is operative between 30 ft and 2450 ft RA. The alerts are for:

Excessive descent rate
Excessive terrain closure rate a. With Flaps NOT in landing position. b. With Flaps IN a landing position.

Altitude loss after takeoff or go-around
Unsafe terrain clearance when not in the landing configuration a. Landing Gear NOT down AND Flaps NOT in landing position. b. Landing Gear NOT down OR Flaps not in landing position.
Excessive deviation below an ILS glide slope
Altitude Callouts
Windshear DetectionThe GPWS takes Input from:

Air Data System
RAD ALTFor Look Ahead Terrain Alerts additional Inputs from;

NON Pegasus FMC
Pegasus GPSNotes

Monitors Terrain using an INTERNAL WORLDWIDE DATABASE. It does not account for manmade objects.
Terrain display is generated using a build in ‘sweep’ on the display. It IS NOT a radar display. This display is computer generated from the database in the GPWS and correlated to GPS position.
Altitude callouts are made based on the setting of the Captains Rad Alt.
Does not provide an alert for flight toward vertically sheer terrain or slow descents into terrain while in the landing configuration.
Uses WGS-84 data(be aware not all countries are WGS-84 compliant) Terrain ahead of the airplane may exceed available climb performance. A ground proximity caution or warning does not guarantee terrain clearance.

Can be more easily described by breaking it down into two distinct parts.

Predictive Windshear.Windshear
Windshear alerts are available during takeoff, approach and landing. GPWS provides a warning when the airplane is in a windshear.

Excessive windshear at the current airplane position detected by GPWS.
Enabled below 1,500 feet Radio Altitude.GPWS Windshear detection begins at rotation.

Predictive Windshear

Weather radar provides “predictive windshear alerts” for excessive windshear ahead of the airplane and these are annunciated on the HSI.

The weather radar

Uses radar imaging to detect disturbed air prior to entering a windshear.
Provides windshear alerts for windshear events containing some level of moisture or particulate matter.
Detects microbursts and other windshears with similar characteristics.
Does not provide alerting for all types of windshear. Note: The automatic function of the GPWS provides “pop-up” displays for predictive windshear and terrain. If neither pilot has weather radar on, and a predictive windshear caution or warning occurs, the weather display and predictive windshear symbol automatically show on the HSI. If neither pilot has terrain display on, and a look-ahead terrain caution or warning occurs, the terrain display will automatically show on the HSI.



Windshear close to and directly ahead of the airplane detected by the weather radar.
Enabled during takeoff, below 1,200 feet Radio Altitude.Go Around Windshear Ahead

Windshear within 1.5 miles and directly ahead of the airplane detected by the weather radar.
Enabled during approach, below 1,200 feet Radio Altitude.Monitor Radar Display

Windshear within 3 miles and ahead of the airplane detected by the weather radar.
Enabled during takeoff and approach, below 1,200 feet Radio Altitude.The weather radar automatically begins scanning for windshear when:

Thrust levers set for takeoff or
In flight below 2,300 feet Radio Altitude (predictive windshear alerts are issued below 1,200 feet Radio Altitude).Alerts are available approximately 12 seconds after the weather radar begins scanning for windshear. Predictive windshear alerts can be enabled prior to takeoff by pushing the EFIS control panel WXR switch.

If windshear is not detected, weather radar returns show only after pushing the EFIS control panel WXR switch.

During takeoff and landing,

New predictive windshear caution alerts are inhibited between 80 knots and 400 feet Radio Altitude.
New warning alerts between 100 knots and 50 feet Radio Altitude. These inhibits do not remove existing predictive windshear alerts.

5th Feb 2010, 11:40
Water in Liquid form has to be present, so on wet snow you will have Predictive WS, Wet Hail....

GPWS gets its input from Number 1 side (Captain's side) so radio altimeter 1 fail you will loose some functions, when on the ground, swapping Radio altimeters will enable GPWS to be back watching :ok:

5th Feb 2010, 11:46
........ swapping Radio altimeters will enable GPWS to be back watching.

Not necessarily. There's more to RadAlts than a black box. Degraded antennas etc etc.

5th Feb 2010, 12:06
radio altimeter 1 fail where is antenna mentioned:ouch:, I really dont understand why there is always someone reading things that are not written...:}

5th Feb 2010, 12:28
Someone else explain, please. My English is, apparently, limited. :hmm: