|Syllabus item 2.7.1 a to g.
Ground Proximity Warning
From the accident files
Case study No.1
A B767 crew departs an airport not fully understanding their airways clearance, and the
air traffic controller does not challenge their incorrect readback. The crew enters the
first report point into the flight management computer (FMC) rather than the standard
instrument departure (SID) which they were to supposed conform to. After takeoff at 750
feet, the crew turns the aircraft to proceed directly toward the first report point, which
is 150 nm from the departure airport.They retract the gear and flaps and accelerate the
aircraft to 250 KIAS. Less than two minutes into the flight, the GPWS warnings begin:
"Terrain, Terrain ! Pull up ! Terrain, Terrain !" The first officer responds
with a gentle pull up from 9.3 degrees to 12.5 degrees. After gaining 200 ft, he lowers
the nose to 11.2 degrees, just before the left wing clips the last 20 ft of a 300 ft
uncharted tower- on top of a 3, 000 ft high mountain ! After the incident, the crew raises
the nose to 16.9 degrees and applies full thrust.
The aircraft returns to its departure airport without further incident. The left
wing has sustained a 1.8 metre
long, 0.6 metre deep hole in the left wing leading edge, a ruptured fuel tank, damage
to the flap drive, stringers and front wing spar, as well as scar of the towers red
paint across the top of the wing.
Case study No.2
The B727 is on an 18 nm final approach for a daylight landing. The flaps are set to 25
degrees and the landing gear is up. The aircraft is fitted with GPWS and a Traffic
Alert and Collision Avoidance System
(TCAS). During the last portions of the approach there are numerous TCAS warnings that
add to the busy cockpit routine of check-lists, announcements and radio communications in
the high traffic density area.
As the aircraft passes 500 ft AGL, the GPWS calls out "Pull Up, Pull Up !"
The Flight Engineer silences those warnings by pulling the circuit breaker. The crew
review the rate of descent, glideslope indications, and flap position as they continue to
descend toward the runway. The aircraft behind notices something strange and reports this
to the tower controller.
At 50 ft, the tower tells the crew to "Go-Around", but it is too late, the
aircraft strikes the runway with its landing gear in the up position. It slides
along the runway, eventually coming to an embarrassing stop.
The previous two case studies are just a sampling of the avoidable accidents, in which
the aircrew disregarded the warnings given by the GPWS. In both cases there was no loss of
life. Circumstances are not always so forgiving.
With the growth of air transport operations after World War Two, there was an alarming
increase in controlled flight into terrain (CFIT) accidents, where a perfectly good
aircraft was inadvertently flown into the ground. To counter this trend a Ground Proximity
warning system was devised, and introduced in air carrier aircraft in 1974. After that
time there was a dramatic decrease in these type of accidents. The rate of CFIT accidents
is still alarmingly high as a proportion of total accidents though. In air transport jets
and turboprops, more than 50% of all accidents involve CFIT. The figures are even more
alarming if you consider hull losses in the corporate jet sector, where about 72% of all
accidents involve CFIT.
Modes of operation
There are six different types of protection given, dependant on the conditions the
aircraft is subject to. These are called modes, and they are ...
|Mode 1 (Refer fig 1)
Excessive rate of
descent with respect to the aircrafts height above the ground. Two different warnings can
be given. First, an advisory (sometimes called a soft warning) of "SINKRATE",
repeated every 3 seconds, and illumination of the amber GRND PROX light. This can be
inhibited by pushing the GND PROX switch. Second, an aural warning (sometimes called a
hard warning) of "PULL UP", together with illumination of the "Pull
Up" light.Both alerts stop when the aircraft exits the respective warning envelopes
(ie: reducing rate of descent to one outside of the warning envelopes).
Fig 1. Mode 1 Excessive descent rate.
|Mode 2 (refer fig 2)
This supplies warning protection when the
terrain is rising dangerously fast. These warnings are given well ahead of the aircrafts
projected collision with terrain. A gain in barometric altitude is required to stop the
alert. Mode 2 is in fact split into two separate sub-modes, mode 2a being if the flaps are
NOT in the landing position, and mode 2b if they are in the landing position.
Mode 2a has an advisory aural of "Terrain, Terrain", coupled
with the illmination of the GND PROX G/S INHB switch light. This can be inhibited by
pushing the GND PROX G/S INHB switch.
If the aircraft radio altitude, speed and rate of closure with the ground are within
the warning envelope, an
aural message of "Pull Up, Pull Up", preceded by a whooping sound will
be heard. This warning can NOT be inhibited.
Mode 2b provides monitoring when the flaps are in the landing position. Entering
the envelope with the landing gear extended would cause the repeated message of "Terrain,
Terrain", coupled with the illumination of the amber GND PROX G/S INHB switch
light. Can be inhibited by pushing the GND PROX G/S INHB switch.
Fig 2. Mode 2a/b. Excessive rate of closure with terrain.
|Mode 3 (refer fig 3)
This mode warns pilots of an
excessive altitude loss after takeoff or go-around. Mode 3 monitors the amount of radio
altitude gained. If the barometric altitude lost equates to approximately 10 % of the
radio altitude gained, the "Dont Sink" aural warning will sound,
coupled with an illumination of the amber GND PROX G/S INHB switch light.
A second aural advisory of "Too Low Terrain" will occur if the
original radio altitude is greater than 150 ft AGL, and the radio altitude then decreases
by more than 25% of that radio altitude. As with a "Dont Sink" aural, the
amber GND PROX G/S INHB switch light will illuminate.
Fig 3. Mode 3. Altitude loss after takeoff or
|Mode 4 (Refer Fig 4)
This is divided into
two submodes, mode 4a, and mode 4b. Both identify to insufficient terrain clearance
during the climbout, cruise, descent and approach phases of flight. This protection is
especially valuable when the aircrafts flight path is too shallow to develop excessive
closure rates with terrain (Mode 2), or excessive descent rates (Mode 1).
Mode 4 has three different alerts, depending on the phase of flight and configuration
of the aircraft.
A caution when the aircraft is near to the ground, and the landing gear is NOT down, or
the flaps are NOT in the landing position. Aurals will be either "Too Low
Gear", "Too Low Flap", or "Too low Terrain", dependant
on aircraft speed. The voiced warning will be repeated until the flight condition is
corrected. Once the landing gear and flap are set to the landing position, the aurals will
Fig 4. Mode 4. Unsafe terrain clearance with
landing gear/flaps not in landing position.
Mode 5 (Refer Fig 5)
This concerns itself with an excessive deviation below the glideslope whilst
engaged in an ILS approach. It will be be active whenever the runway being approached is
equipped with an instrument landing system, and the aircrafts navigation radio is tuned to
the correct ILS frequency. In other words, mode 5 is not going to provide a caution if
your navigation radio is tuned to a VOR, whilst engaged in a VOR approach.
This aural warning will warn the flight crew if the aircraft descends to a position 1.3
dots or more below the ILS glideslope. As mentioned, mode 5 has two volumes, soft and
loud. The repetition rate is increased as the
deviation from the glideslope increases, and radio altitude decreases. The aural
warnings are coupled with an
illumination of the amber GND PROX G/S INHB switch light.
This mode is only armed when a valid ILS signal is being received, the radio altitude
is 1, 000 ft or less, and
the landing gear is down.
The envelopes have two aural warning volumes, both of which are "Glideslope".
This aural warning will warn the flight crew if the aircraft descends to
a position 1.3 dots or more below the ILS glideslope. As mentioned, mode 5 has two
volumes, soft and loud. The repetition rate is increased as the deviation from the
glideslope increases, and radio altitude decreases. The aural warnings are coupled with an
illumination of the amber GND PROX G/S INHB switch light.
Fig 5. Mode 5. Below ILS glideslope.
The GPWS provides aural and visual warnings of significant windshear conditions. The
aural warning consists of a two tone siren, plus the words "Windshear, Windshear,
Windshear". The aural warning is given once only during a windshear encounter,
and is associated with the illumination of the master warning "Windshear" light.
Some EFIS equipped aircraft have a feature which gives pitch and wing levelling guidance
to pilots through the flight director bars on the ADI. Refer fig 6, which is the display
on a B767 aircraft ADI. A "Windshear" anninciation will also appear on the ADI
of the B767 aircraft.