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5% 10% 15% 20% 25% 30% 35% 40% were confused with another, leading to an altitude
Percentage of Total Altitude Deviations overshoot or undershoot on either climb or descent.
8 ASRS Directline
…Or Was That Eleven Ways?
Too Many Numbers
Controllers include several (sometimes, too many)
numbers in the same radio transmission.
“The controlling agency, in rapid manner, told us to
turn to 310 degrees, slow to 210 knots, and I understood
him to say ‘maintain 10.’ ”
“Very often controllers issue four to five instructions
in the same breath, such as ‘turn left 330 degrees,
maintain 2000 feet till established, cleared for ILS
30 approach, contact tower 119.4 at the outer
marker, and maintain 160 knots until five mile
final.’ ”
Similar Numbers
Altitude crossing points stated in miles may be similar
to the altitude to which the flight is cleared.
“Were we cleared to 10,000 feet 11 miles west of
ARMEL, or 11,000 feet 10 miles, or 10,000 feet 10
miles, or 11,000 feet 11 miles?”
“Center cleared us to cross 10 DME NE PVD 11
thousand, 250 knots. I read back 11 miles NE PVD
10 thousand, 250 knots. At 10,100 feet I questioned
center, and they said 10 north east at 11 thousand,
250 knots. We climbed back up to 11,000 feet.”
250 Knots at 10 Thousand
Pilots tend to associate a 250 knot speed restriction
with a 10,000 foot altitude assignment, since civil
aircraft are normally restricted to a speed of 250 knots
or less below 10,000 feet.
“A clearance for 250 knots generally makes a pilot
think about 10,000 feet due to the association of 250
knots below 10 thousand.”
“We think the 250 knot restriction could have led us
to assume 10,000 feet because the majority of locations
use 10,000 feet/250 knot crossings in their
STAR’s [Standard Terminal Arrival Routes].”
Spring Loaded
Pilots may anticipate receiving a certain clearance, but
get something just a little different. Perhaps the last
SID or STAR they executed had speed and altitude
crossing restrictions that were similar, but not exactly
the same as the one they are currently flying.
Noted an air carrier pilot who initiated a premature
descent to 10,000 feet from 11,000 feet: “I may have
anticipated being given 10,000 feet after seeing [an
air carrier aircraft] pass below me.”
Failing to Question the Unusual
Pilots may, or may not, be familiar with normal ATC
procedures in a particular area, and in either case,
neglect to question an abnormal altitude assignment.
“Next time in and out of DEN we will be aware that
the inbound aircraft are normally at 11,000 feet and
departure aircraft normally restricted to 10,000 feet.”
“The usual clearance for this arrival is 11,000, but
we both followed my error blindly to 10,000 feet.”
The Ten Mindset
Pilots and controllers get what is referred to as a
“number ten mindset” after hearing a lot of zeros. It
seems like one-zero-thousand then becomes the
altitude assignment.
“I do think the number of tens in the clearance was a
contributing factor.”
“Flight crew read back ‘one-one-thousand’, but
somehow had mindset of one-zero-thousand.”
Reduced Monitoring
Cockpit duties and distractions result in only one flight
crew member monitoring the ATC frequency. Similarly,
controller workload and frequency congestion are
factors which affect the ability of controllers to closely
monitor pilot readbacks.
“This type of situation has occurred with this crew
member 3 or 4 times since flying two man crew
aircraft when one crew member is busy reviewing
approach plate and procedures and is distracted
from hearing conversation between [the] other crew
member and controller.”
ASRS Directline 9
One Zero Ways to Bust an Altitude continued…
Cockpit Management
Cockpit management and flight crew coordination
may be less than optimum, and crew
members fail to adequately monitor each other
in such tasks as altitude alert setting or readback
of clearances.
“Center cleared our flight from 17,000 feet to
11,000 feet MSL. This was acknowledged by
me, however the first officer understood
10,000 feet and placed that altitude in the
selector.”
“I will have to watch the music closer while
the other guy is playing the piano.”
Radio Technique
Very often controllers and/or pilots fail to use
proper techniques. I consider this to be the
　
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