Definition
Flying by relying on bodily sensations -- the feel of acceleration, tilt, and pressure transmitted through the seat and harness -- rather than on the flight instruments. In instrument flying this is treated as an error, because the inner-ear and body senses are unreliable when there are no outside visual references and will routinely produce false impressions of pitch, bank, and turning.
Plain English
Trying to fly by what your body feels -- the way the seat pushes on you, which way you feel tilted, whether you feel like you are climbing or turning -- instead of believing what the instruments are telling you. Without an outside view to check against, those bodily feelings lie, so pilots are trained not to trust them.
Context Anchor
Seen in instrument takeoff training, especially when discussing the error of trusting body feel after takeoff instead of the instruments.
Derivation
An old expression from early aviation, when open-cockpit pilots literally felt the aircraft through the pressure of the seat against their body. It survives in aviation today as shorthand for 'flying by feel alone' -- which is exactly what instrument pilots are taught not to do.
Why Pilots Care
Using seat-of-the-pants cues in instrument conditions often produces spatial disorientation and loss of control.
Intuition Check
Seat-of-the-pants does not mean good pilot judgment here. It means body-feel cues, and in instrument flying those cues can be misleading.
Example Sentence 1
The instructor cautioned the student against flying seat-of-the-pants once they entered the clouds, and told him to keep his eyes on the attitude indicator.
Example Sentence 2
During the instrument takeoff briefing the instructor reminded the crew that seat-of-the-pants flying must be ignored once the airplane enters the clouds.