Monday, April 28, 2014

The Ground Detector

Let me start by giving full credit for this information to Mr. Archie Trammell.  This remarkable man is the master of Weather Radar.  These techniques, I discuss, I learned from one, solitary, magazine article he wrote.  Thank you sir, for I have used these methods on every flight for over twenty years.
William (Billy Howe  

The Under Utilized Weather Radar

Every pilot in our group instinctively turns on the weather radar during the spring through fall days when thunderstorms are likely to be present.  Otherwise we mostly leave them off through the winter and on fair days.  Consider this question.  What other hazard to aviation could we detect with the use of the radar that is present on every single flight?


Many of us have sat through a training film on Controlled Flight Into Terrain (CFIT) several times.  We all know of the sad and tragic ends to the Beechcraft King Air that hit a ridgeline, at night in VFR conditions in Canada; the Gulfstream Jet that hit the fog shrouded hilltop in Kuala Lumpur; and the Beechcraft Jet that hit another cloud covered hilltop near Rome, Georgia.  I would submit other tragic accidents for consideration.  In December, 1974 a TWA Boeing 727 hit that ridgeline northwest of the Washington Dulles airport during a non-precision approach to runway 12.  In March, 1991 a Hawker Jet carrying eight members of country western singer Reba McEntire’s band hit a ridge, at night, near San Diego, CA. On December 20, 1995 an American Airlines Boeing 757 struck a ridge near Cali Columbia, at night.  Every one of these aircraft had the means of knowing that they were in close proximity to the rising terrain – the weather radar.

To Be Forewarned

I am very glad of the Enhanced Ground Proximity Warning System.  It is a lifesaver.  However, which is better, to see a hazard from a safe distance and maneuver to safely avoid conflict, or to blindly trust the warning system and react to a sudden and startling and last second “LOOK OUT!”, or in our case, “PULL UP! PULL UP!”  I cannot cite accurately this last example since I only heard about it through the aviator’s grapevine, but I heard a story of an Express Jet EMB-145 on a recent approach to a destination in Mexico.  This flight crew was so startled by the EGPWS sudden command to “PULL UP!” that they basically did nothing and wrote-up the system after landing as malfunctioning.  Imagine their shock to learn that investigators determined that they came within a few hundred feet of oblivion.  So it is, we have the on-board radar to paint a picture, but we do not use it and often have no clue where the rocks are.

Height Evaluation Technique

Using the tilt control to determine the aircraft height above the ground is easy.  The magic formula is:
Height ∆ = Tilt ∆ X Miles X 100
This reads: Height change equals tilt change times miles times 100.  If we start with the tilt set at the park position, that is, at a tilt setting wherein the bottom of the beam is parallel to our flight path then we know that anything reflecting on the screen is at our altitude or higher.  I like this position as I approach the ground because as I descend into the valleys I can “see” the surrounding terrain as it rises above me.  For us EMB-145 guys this park position is almost always 7°.  From this starting position we can lower the tilt, typically, by four degrees to a position of 3°.  As you descend from the cruising altitudes, this “down four degrees” position will show ground returns up to the 25 mile arc as you get within 10,000 feet of the ground.
10,000 ∆ = 4°∆ X 25 miles X 100
As you get closer to the ground and bring in the range, you will see the ground 5,000 feet and below you at the 12.5 mile arc and most telling, 2,000 feet and below at the 5 mile arc.  While maneuvering for an approach, if any ground return penetrates that 5 mile arc I am immediately concerned.  As I cross the FAF and descend for touchdown I will go back to 7° so as to look for terrain above me beyond the airport.  An additional bonus to the “down four degrees” position is detecting wind shear probability.  If there was a strong down burst on final, near the airport, that column would likely attenuate your radar returns and show you a shadow.  Rather than a smooth, uniform ground paint, there is a V shaped hole.  We all know not to even go there.  This brings in another interesting question.  What is the Ground Mapping setting about?  The manuals are very vague on that one.  I believe I have found the functional benefit of the GM feature.  As we approach the ground while rain is in the area it becomes difficult to discern the ground returns from the rain returns.  Selecting the GM setting will automatically reduce the gain to a level where the rain disappears while still returning ground images.  Thus, I find myself going back and forth between WX and GM so as to get the total picture.

Park Position Test

How do we know that the park position is 7° in these EMB-145 aircraft?  Well, you don’t until you test it out on the specific aircraft you are flying and this test works for any radar in any aircraft type.  Recently I flew with a radar set which indicated 9° as the park position.  That unit was also having stabilization issues so I wrote it up.  As you are cruising in level flight lower the tilt so as to paint the ground up to the 25 mile arc.  Now divide your height above the ground by the (25 miles X 100).  This will give you the number of degrees to raise the tilt from this present position so as to bring the bottom of the beam to your level.  A typical example is while cruising at FL360 we roll the tilt down to paint ground right up to the 25 mile arc.  The terrain is about 800 feet MSL so we will just consider ourselves as being 35,000 feet above the terrain.  (35,000 feet/ 25 miles / 100 = 14)  (Hint: You can shortcut all those zeros and just divide 35 by 25.  The answer will be 1.4, but we know that is really 14) Now we note that the present position of the tilt is -7°.  When we add 14°to the -7° we get 7° as the park position.  Most of the time you will get a non-whole angle change value such as 13.4°; just round to the nearest whole number.

Be Safe!

As professional pilots we must utilize every suitable means necessary to remain situationally aware.  Using these radar techniques are simple and so valuable especially while operating near the ground on “dark and rainy nights”.

Captain William (Billy) Howe
June 7, 2013


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