The
"parallax effect" describes a type of visual illusion in which
the position of an object in 3-dimensional space appears to change, due
to a shift in the position of the observer. The parallax effect can make
distant fixed objects, such as a planet or star, appear to be close and
in motion. The twinkling planet Venus is a well-known example in aviation.
Tower controllers have often cleared Venus to land, while pilots have
mistaken the planet for nearby aircraft position lights.
The parallax effect is especially
apt to occur during night operations when there may be few, or no, visible
references to the horizon as an aircraft moves through space. Several
ASRS reports illustrate, beginning with a First Officer’s account
of a nighttime evasive maneuver that startled crew and passengers:
- I observed
what I believed to be an imminent traffic conflict. I manually overrode
the autopilot and started an immediate left turn. The perceived conflict
was a result of slight parallax of green and red wingtip lights of
another aircraft. A bright white star also appeared as one of the
running lights on the perceived conflict. The maneuver was a gut reaction
on my part, as I perceived the aircraft to be within a few thousan
Color d feet from us. Passengers and flight attendants who were not
seated with their belts fastened were upended in the cabin. One passenger
received an abrasion to a knee and one complained of a neck injury...
After landing...[no passengers] required medical attention... The
aircraft was inspected for overstress and no discrepancies were found.
A conservative approach,
followed by the First Officer in this instance, is to avoid the perceived
hazard first, and verify the nature of the hazard afterwards. Although
this report didn’t mention crew fatigue as a factor, fatigue is
known to be associated with susceptibility to the parallax illusion.
U.S. Air Force research has shown that a few minutes of breathing 100%
oxygen will help to refocus pilots’ thinking—and eyesight.
The parallax effect also
can be experienced by several observers at the same time, as reported
by a general aviation pilot who described a night flight with companions:
- There were
3 of us in the cockpit, including 1 non-pilot. We were heading northbound
over the peninsula... All 3 of us looked off to the 9 o’clock
position and saw 2 landing lights which appeared to be a single large
aircraft approaching at a very rapid rate... We banked to the right
and pulled the throttle to idle in an attempt to avoid what appeared
to be an imminent collision course. We then returned to level flight
to see that the perceived aircraft was still approaching from the
same direction and was now much closer. After another brief moment,
we realized that what we were looking at was the landing lights of
two separate aircraft approaching from the west and that we were experiencing
a visual illusion. It was not until the aircraft finally got close
enough to see the position lights that we were able to distinguish
one aircraft from the other.…
The reporter noted that contributors
to the event were a very dark night with no moonlight, and the aircraft’s
proximity to the ocean with its lack of surface lights and features.
Awareness of the flight conditions conducive to the parallax effect
can help keep pilots from falling victim to this illusion.
The August 1999 CALLBACK
contained an article about a baggage handler who went to sleep in an
airliner cargo hold and awoke at FL200. His banging in the cargo hold
was heard by the cabin crew, and the aircraft made an emergency landing.
But what happens when the cries for help of a cargo hold occupant can’t
be heard? As this First Officer’s report concludes, the occupant
may be lucky to survive:
- I was the
First Officer on a cargo/mail flight from XXA to XXC, with a 16-minute
stop in XXB. The first leg was flown at FL310 and the second leg at
FL270. During post-flight duties at XXC, our company mechanic came
to the cockpit and advised there was a foreign national in our aircraft
cargo compartment, cold but alive, stating that he was a company employee
that had been closed in the aircraft belly compartment in XXA. Maintenance
also advised the airport security and an ambulance were enroute.
The man appeared cold but otherwise OK. He said he was waiting in
the aft belly for cargo to be loaded. No cargo arrived. The door was
closed. Due to the ramp noise, hearing protection, and the APU I can
only assume no one could hear his cries and knocking for attention.
It is the responsibility of the loading crew to look inside the compartment
before closing the door... It is my understanding that my company
and the FAA are looking into ways to prevent this from happening again...
If this had been a much longer flight, I am sure it could have been
fatal. As it was, the man was closed in the compartment for a total
of 2-1/2 hours; 1/2 of that [time] was in flight.
Clip-on badges or flags of
a distinctive color, attached to the cargo conveyer belt, would be an
effective way of signaling that the cargo hold was occupied. Removal
of the badges/flags could be a ground crew check item.
An article in the October
1999 CALLBACK on battery fire hazard has created a ripple of interest,
and several letters to the Editor. We’d like to share excerpts
from this correspondence with readers who may carry spare batteries
in their flight gear or personal belongings. We begin with a pilot’s
tale of a battery explosion in flight:
- Just wanted
to reinforce the problem with carrying batteries with unprotected
terminals as described by the C-172 pilot in the Oct. 1999 CALLBACK.
I was climbing out on takeoff years ago in a C-182 when I heard a
sharp explosion. Turned around and saw a cloud of light smoke in the
baggage compartment.
I thought the aircraft battery had exploded but alternator output,
etc. were normal. Immediately returned to airport, landed, and removed
everything from baggage compartment.
After checking aircraft battery, began to empty luggage, etc. Finally
found the remains of the culprit in my flight case. I was using a
portable intercom at the time and had changed batteries (9V alkaline)
just before engine start and had thrown the "discharged"
battery into my flight case for disposal at home. It had shorted out
on a metallic piece of the flight case and had exploded, apparently
due to the ignition of a gaseous by-product of the shorted condition
which accumulated in the flight case….
If this explosion had occurred in a pocket, it could have caused serious
injury. My wife is a Registered Nurse, and was in the habit of carrying
spare batteries for pacemakers, etc. around the hospital in a pocket
with coins, keys, etc. Needless to say, she immediately stopped that
habit.
When we carry 9V alkalines now, they not only have a terminal protector
in place, it is also held on with rubber bands.
Hope this word of caution helps keep the skies (and our bodies) safer.
Another reader adds a domestic
note on battery-related "hot pockets" that has implications
for many light-airplane pilots—and their passengers:
- Recently,
I was changing the batteries in my home smoke alarms, and upon removal
of an old battery, I put it in my pocket... In short order I realized
that I had a "hot pocket"... I reached into my pocket to
remove the hot material and discovered the old battery, which was
very warm. I then realized that it had shorted out upon contact with
either the foil of my life savers or the steel in my pocket knife...
Being a pilot, your article alerted me to the fact that I must now
treat batteries with special respect while having extras aboard...
A
summary of these battery care "life savers."
- Use
terminal protectors on all batteries carried onboard an aircraft,
or leave new batteries enclosed in their original protective wrapping.
- Secure
terminal protectors with rubber bands or other devices that won’t
themselves cause a fire hazard.
- Don’t
place unprotected spare or used batteries in pockets, flight cases,
or other enclosed spaces that contain metallic objects.
A general aviation pilot
recently supplied ASRS with a compelling tale of "wrong way"
ground navigation:
- I had flown
into [airport] for the first time two days prior [to incident]...
The Ground controller gave me excellent progressive taxi instructions
to the general aviation tiedown area.
[On day of incident] I was cleared to taxi to Runway 03 via Bravo
taxiway. Ground instructed me to follow the taxiway out of GA parking,
and turn right at Bravo, which I did. I was unable to see a separate
run-up area, so upon reaching Runway 03, I stopped behind the runway
boundary, switched to Tower frequency, and began my run-up. Tower
called...and said that I was blocking the taxiway...and told me I
should move to the run-up area. I turned the aircraft around, pointing
it now at the side of the taxiway away from the runway and asked if
the direction I was now pointing was the direction of the run-up area
(it was a wide taxiway, and I thought the far side might be the run-up
area). Tower told me "No, just go to the end of Runway 03."
I thought it was an unusual place for a run-up, but I visually confirmed
that there were no aircraft on final for Runway 03, and the Tower
frequency was congested, so I simply responded "End of Runway
03."
As soon as I was on the runway, Tower called and asked if I had entered
the runway—evidently surprised that I had. I responded that I
thought that’s what he had told me to do. He responded that he
hadn’t... In discussion afterwards...[my passenger and I] concluded
that the controller had meant that we should have gone to the extreme
southern edge of the taxiway adjacent to the end of Runway 03.
The situation could have been avoided if: 1) I had asked Ground about
the specific location of the run-up area; 2) Tower had indicated "the
taxiway adjacent to the end of Runway 03" instead of "the
end of Runway 03"; 3) I had called for confirmation on what I
thought was an unusual instruction.
Tower controllers, as well
as pilots of large jet aircraft, have a better overall view of runways
and taxiways than do light airplane pilots. ATC should keep this in
mind when giving taxi instructions. Pilots of light airplanes should
ask for progressive taxi instructions when uncertain of directions.
