The Low-Down on Altimeter
Settings
 ASRS
recently received three reports from three separate operations, all occurring
on the same day and referring to the same situation--low barometric pressure
at an Alaska airport. Although these incidents occurred at a single northerly
location, they offer a reminder to pilots in other parts of the country
that the season is not past for unusually low barometric settings. They
are also of interest because of several misconceptions expressed by reporters
about ATC/FSS responsibilities in regard to altimeter settings. Here are
excerpts:
- ATIS [reported
altimeter] 28.84. No mention of low altimeter was made. Center cleared
us [up] to FL330. Leaving FL180, altimeters were set to 28.92 Captain
and 29.92 First Officer (F/O). At FL320 Captain's altimeter, I called
FL320 for 330. The First Officer called 330 noticing the wrong altimeter
setting on my side. I immediately leveled off and descended to FL330...while
resetting my altimeter to 29.92. In the future, I intend to be much
more careful when resetting and cross-checking the altimeters, especially
when low altimeter settings are reported. I feel this mistake might
have been avoided if the ATIS had mentioned the low altimeter setting.
In recording the ATIS, some controllers may emphasize the altimeter setting
by stating, for example, "a low 28.84." Some Flight Service
Station briefers also adhere to this practice. However, this procedure
is not mandatory.
- Destination
weather [reported altimeter] 28.83. Prior to initial descent, the Second
Officer received and put the ATIS information on the landing bug card,
except the altimeter was written as 29.83... The Captain started [a]
go-around at the same time the Tower reported they had a low altitude
alert warning from us... ATC does use the term low/low after low altimeter
settings. At what setting it is required, I don't know, but I feel any
time it is below 29.00, it should be used.
Again, there is no requirement
for controllers to notify pilots of unusually low barometric conditions,
although many controllers elect to do so. The phrasing "low/low"
is a technique used by some controllers to emphasize a particularly low
altimeter setting, but pilots shouldn't count on hearing it.
- We departed
[airport] where the local altimeter setting was 28.84... About 15 minutes
after reaching cruise altitude FL410 the copilot noticed we had set
28.92 rather than 29.92 at FL180. We reset the altimeter...and descended
to FL410... In over 30 years of flying, this may have been the tenth
time that I've had an altimeter setting below 29.00. We are careful
in setting the hundredths portion (.92) of the altimeter, but need to
consider the total setting (29.92). It can jump up and bite you occasionally.
This Captain recognizes the
bottom line: it is the flight crew's responsibility to ensure correct
setting of the altimeter, and to maintain good cockpit communication to
catch any errors.
High to Low, Look Out
Below!
A general aviation pilot flying
in the Great Lakes region encountered an extreme instance of barometric
pressure changes. His story also illustrates why it's important for pilots
to note significant changes in barometric pressure readings during preflight
checks of weather along an intended flight route:
- Southeast
bound [on airway] at 17,000 feet indicated altitude, controller reported
my altitude encoder indicated 16,000 feet on the readout. I had departed
VFR and picked up my IFR clearance at about 4,000 feet... I had set
the barometric pressure as provided by Center when clearance was provided.
I was approaching a cold front which was lying north to south over Lake
Michigan. The controller asked if I had a backup encoder. I said no,
and asked for an altimeter setting. The setting provided was 1 inch
lower than the previous provided setting (about 100 nm earlier). I reset
my altimeter... After the reset my altimeter now indicated 16,000 feet...
The problem was evidently a very steep pressure gradient behind an approaching
the [cold] front...
A Toss and a Catch
Altimeter setting mishaps are
more often due to human performance fluctuations than the barometric variety.
Here's what happened to a flight crew that forgot to "wipe the slate
clean":
- The engineer
threw the ATIS up onto the center console (or perhaps he handed it up
to me and I put it there). The altimeter on the ATIS called for 30.17
When we set this in up front and ran the approach check, the sharp engineer
remembered that he had copied down 29.67 and brought this to our attention.
We had turned the ATIS sheet face up rather than the arrival ATIS which
he had copied on the [back]. Weather at our destination was 300 feet,
and the difference in altimeter settings between 30.17 and 29.67 was
500 feet. Had this not have been caught by the engineer or later through
ATC, the results might have been disastrous. This could be the result
of having the company departure/arrival ATIS on the same sheet...
"Practice in Little
Things, Proceed to Greater" 
Springtime usually brings an increase in flight activities, and with it,
the challenge to instructors of supervising many small but vital cockpit
details. Two flight instructors relate how neglect of small matters led
to significant consequences.
- My student
and I took off for a night flight in the local practice area. We were
level at 2,000 feet when the engine quit... My first physical action
was to take the controls, pull out the carb heat, check mixture and
fuel, and I told my student to take up the checklist and go through
the restart procedures while I'm flying the airplane and looking for
a field... At approximately 600 feet, I called out again to my student
to check the prime and ignition, and I checked the carb heat, mixture,
and fuel. No start. I started to shut down the engine. I pulled the
mixture, took the fuel selector towards the OFF position, and same with
the ignition switch. Something was wrong with that picture. It probably
took a few seconds before I realized that the ignition switch was in
the OFF position. I reached over and twisted it to BOTH, pushed the
fuel back on, and the mixture in. The engine started and I initiated
a climb...
What caused this "emergency?" My student probably must have
hit the ignition switch with his knee, causing it to twist to the OFF
position.. I took for granted that my student would perform the checklist.
Later when I called out for him to check the prime and ignition switch,
which I physically did not check myself, I could see him reaching out
for the items, but I did not realize that he just touched them, not
actually verifying them to be in the right position. I learned my lesson.
Teach students to visually check all items on the checklist, not just
call them out and touch them without even bothering to look at what
they are doing.
In preparation for simulated
emergencies, many students simply memorize the drill, without associating
the checklist words and "make believe" physical actions with
visual verification of knobs and switches.
What a Difference a Letter
Makes
"What a difference a day
makes, twenty-four little hours..." So goes the old song. This instructor
learned that one little letter--as in assure versus assume--can
be important, too.
- On a routine
training flight to practice instrument approaches, we were given an
IFR clearance to "Maintain 5,000, cleared to the [fix] via 12 mile
DME arc and inbound on 117 degree radial." We were not cleared
for the approach due to company aircraft conducting an approach at the
same airport. Immediately after we read the clearance back, the company
aircraft reported his missed approach. The student turned onto the arc
and began descending... After he descended to 4,900 feet, I asked him
if we were cleared for the approach. He replied that we were. At this
point I made several assumptions: 1) We were told to expect approach
clearance after company aircraft completed his approach; I heard him
call missed and assumed we were cleared. 2) Student said we were cleared
and I thought I missed the radio call... At 4,500 feet, Center asked
us to verify our altitude busted!
Reasons: Instructor's failure to verify clearance, (you know you're
in trouble when you state items such as "student said" and
"I thought"), and assuming the student was right. I violated
a cardinal credo for instructors here. Corrective action: Increase communication
between instructor, student, and Controller to assure and not assume
critical items are not misunderstood.
"Mis" Manners
Any radio problem is an inconvenience,
even when a handheld transceiver is on board for backup. But when a bad
radio affects the entire frequency, it becomes inconvenient for everyone.
ATC transmissions may come to a standstill, and other aircraft must return
to a previous frequency or try to contact the next sector's frequency
in an effort to maintain communication. More from this ASRS report:
- Shortly
after takeoff I noticed that I was not hearing anything on my radios.
I tried the second com and switched all possible switches on the radio
panel. I also tried to receive ATIS from two different airports. We
did some sightseeing while I tried to sort things out. After about an
hour, we returned to the field. I remained clear of all restricted airspace,
Class B and C space, etc. Approximately 8 miles out, I called them on
my handheld transceiver and landed without incident. After landing,
I was asked to call the Tower and was told that I had a stuck mike and
was interfering with their communications for 45 minutes after takeoff.
While trying ATIS, I also interfered with that frequency. Better training
on radio communications could have prevented this... Look for a common
denominator--it is very unusual for two radios to fail while navs continue
to function.
Pilots use a variety of techniques
to check for stuck mikes: setting the microphone to "interphone"
to see if the radio clears; reducing squelch; and other techniques that
can help quickly pinpoint the nature of the radio problem.
... and ELT Manners
- While changing
the ELT battery, I inadvertently activated it to the ON position. I
did not turn on my radio to see if it was broadcasting. After about
3 hours, a gentleman from search and rescue came by to turn it off.
What more is there to say,
except that the bill is probably in the mail!
ASRS Recently Issued Alerts
On...
- A BA-41 emergency due to
improper fairing repair
- Recurring inflight airframe
vibrations on the Airbus 320
- HF frequency congestion
between Singapore and Taipei
- BA-31 loss of control attributed
to B-757 wake turbulence
- Concerns about new ATC departure
sequencing equipment
January 1995 Report Intake
- Air Carrier Pilots--1949
- General Aviation Pilots--582
- Controllers--56
- Cabin/Mechanics/Military/Other--17
- TOTAL--2604
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