airplanes equipped with instrument landing systems are vulnerable
to capturing erroneous glideslope signals. Boeing, the International
Civil Aviation Organization, and the U.S. Federal Aviation
Administration are working together to improve awareness and
prevent such errors. Flight crews can help manage the risk
by understanding the problem and performing glideslope confidence
With the advent of instrument
landing systems (ILS) in the 1940s came the possibility of erroneous
or false glideslope indications under certain circumstances. One
such erroneous indication recently occurred on several 767, 777,
and Airbus airplanes, resulting in coupled ILS approaches being
flown toward a point short of the runway. This kind of problem can
occur on any airplane with any ILS receiver.
Boeing has taken action
to help prevent such incidents by revising operations manuals and
working with the International Civil Aviation Organization (ICAO)
and the U.S. Federal Aviation Administration (FAA) to address maintenance
errors that can cause erroneous glideslope signals. The subtle nature
of the indications makes it imperative that flight crews also help
manage the risk by understanding the problem and performing glideslope
This article describes
involving an erroneous glideslope signal.
of erroneous glideslope signals.
INCIDENT INVOLVING AN ERRONEOUS GLIDESLOPE SIGNAL
On the night of July
29, 2000, an Air New Zealand 767 was on a routine flight from Auckland,
New Zealand, to Apia, Western Samoa. The night was moonless, with
scattered clouds that prevented visibility of the runway lights.
The flight crew members
were experienced in conducting routine automatic landing approaches
in low visibility. They considered a routine automatic landing approach
to be safe if the autopilot was coupled to the airplane, no warning
indications were visible, and a valid Morse code identifier signal
came from the ground navigation aids.
Well prepared before
descent, the flight crew thoroughly briefed for the approach. When
the crew selected the approach mode, the glideslope capture occurred
almost immediately. All ILS indications appeared to be correct.
With all three autopilots engaged, the captain concentrated on configuring
the airplane and slowing it for landing. The crew attributed the
slightly steep descent of the airplane to its heavy weight and tailwinds.
The crew noted a good Morse code identifier signal and no warning
indications. At 1,000 ft, the crew completed the landing checks.
Shortly thereafter, the first officer observed the close proximity
of the island lights out his side window. The captain noticed that
the distance measuring equipment (DME) indications differed slightly
from what he would have expected.
The captain executed
a timely go-around 5.5 mi from the runway at an altitude of less
than 400 ft. The crew successfully executed a second approach by
using the localizer and ignoring the on-glideslope indications.
CAUSES OF ERRONEOUS GLIDESLOPE SIGNALS
Investigation of the
Air New Zealand incident revealed important information about the
causes of erroneous glideslope signals. Understanding these causes
requires a discussion of the ILS and its normal operation.
ILS ground equipment
provides horizontal and vertical guidance information to airplane
instrumentation. The equipment typically comprises five components:
a localizer transmission system, a glideslope transmission system,
a DME or marker system, a standby transmitter, and a remote control
and indicator system (fig.
During normal ILS operation,
the localizer and glideslope transmitters each radiate a carrier
wave of 90- and 150-Hz signals of equal amplitude. These signals
alone do not provide guidance but are compared with separate 90-
and 150-Hz sidelobe signals radiated by the localizer and glideslope
to create complex interference patterns. The patterns are designed
so that when an airplane is below the desired glideslope, the instruments
will sense a predominance of 150-Hz signals; when the airplane is
above the desired glideslope, the instruments will sense a predominance
of 90-Hz signals; and when the airplane is on the glideslope, the
instruments will sense equal amounts of 90- and 150-Hz signals (fig.
The ILS was designed
to protect against transmitter malfunctions. If a primary transmitter
malfunctions, the system automatically will transfer to the standby
transmitter. If the ILS does not change over to the standby transmitter,
or if the standby transmitter is faulty, the system automatically
will shut down, and an alarm will sound in the control tower.
It is important to note
that, because the Morse code identifier signal is carried only on
the localizer carrier signal, the flight crew only knows whether
or not the localizer is transmitting. No information on the health
of the glideslope, localizer, or other functions is provided.
On the night of July
29, 2000, the glideslope sidelobe amplifier was not operating in
Apia. In addition, the ILS ground equipment had been left in bypass
mode following calibration maintenance. This prevented system transfer
to the standby transmitter. No alarm sounded in the control tower
because the cable that fed information to the tower navigation status
displays had been cut during construction. As a result, the Air
New Zealand flight received only the glideslope carrier wave transmission,
which was interpreted by the instruments as being on glideslope,
with no warning indications.