A Warning Offer Not Taken
Using the minimum safe altitude warning system
BY J. MAC MCCLELLAN
FOR MANY YEARS AIR traffic controllers’ radar has had the capability
to alert the controller when an airplane he is tracking on radar is
flying too low for safe terrain clearance. The controller then, of
course, passes that alert on to the pilot.
The automated system compares the Mode C altitude reported
by an airplane’s transponder to the known elevation of terrain near
and ahead of the airplane to watch for an unsafe low altitude. The
FAA calls the capability the minimum safe altitude warning
(MSAW), and it is designed primarily to alert pilots flying under
IFR, but it does generate an alert for any aircraft receiving advisories from controllers.
The typical minimum safe altitude for the MSAW system is
1,000 feet above nearby terrain or obstructions. When an airplane
is on a runway approach path, the safe altitude clearance must be
less as the airplane descends to land. And in certain areas designated as mountainous the alert is set to sound when an airplane
drops below 2,000 feet of terrain clearance.
MSAW works very well for pilots flying IFR because assigned
altitudes provide the minimum safe terrain clearance so there are
few false alarms. But MSAW can become a nuisance when a pilot is
flying VFR and providing his own visual terrain clearance. A pilot
may be perfectly happy, and safe, flying closer to terrain under VFR
than the IFR-based MSAW expects. In that situation the MSAW
can annoy both pilot and controller by
warning of terrain or obstructions the VFR
pilot already sees.
THE MOUNTAINS AROUND ASHEVILLE
The Asheville Regional Airport (AVL) is
tucked into a valley near the highest mountains in North Carolina. In fact, the
mountains near Asheville are the highest in
the eastern 48 states, and the MSAW there
can create many nuisance alerts for pilots
flying VFR while receiving radar traffic
advisories. To provide enough time for the
controller to warn the pilot of a low-alti-tude threat, the MSAW system must
“predict” a terrain conflict that is ahead of
the airplane’s track. The automated system
can’t know that a VFR pilot plans to turn
before reaching the mountain ahead, or
that the pilot plans to fly between the peaks
with heading changes to remain clear, so
MSAW issues an alarm that is a distraction
more than a true safety aid.
