Our flight was delayed from takeoff because the captain reported it to be "out of trim". What does this mean?
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2$\begingroup$ Did you notice people being shifted to different seats during the delay? For center of gravity purposes, the cabin is divided into zones. It's possible that the actual assigned seats filled left one or more zones "underweight" you might say, due to ppl not showing up. If this was the case you'd have seen the Flight Atts requesting people to move to different seats. $\endgroup$– John KCommented Nov 9, 2018 at 20:54
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3 Answers
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In this context, it would mean the aircraft is loaded in such a way that the Center of Gravity is too far forward or aft. That's actually not the way we usually use the word "trim" in aviation but it's what it means in this case.
answered Nov 9, 2018 at 19:56
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1$\begingroup$ How is this detected on the ground? are there dedicated sensors or a system for that? $\endgroup$– AlexCommented Nov 9, 2018 at 21:44
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17$\begingroup$ @Alex: That sounds like a great candidate for posting a new question. $\endgroup$ Commented Nov 9, 2018 at 21:51
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4$\begingroup$ @Alex: they know a lot about the baggage, and they know where the passengers sit. That's why they will move passengers around. If you fly on a small enough plane, they will ask you your weight (it's happened to me twice, though once was a large passenger helicopter). $\endgroup$– Flydog57Commented Nov 10, 2018 at 1:07
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1$\begingroup$ aviation.stackexchange.com/questions/1850/… $\endgroup$ Commented Nov 10, 2018 at 18:31
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"Trim" is the ability to correct for deviations in flight controls by prepositioning the flight surfaces for a particular direction. For example, if the aircraft is pitching up a bit by default, you can trim down in order to counteract it.
"Out of trim" would mean that the Aircraft has either been improperly trimmed, or is operating outside of the range trim can be applied. (If the aircraft is pulling down hard, you can only trim up so much.) This can be due to a mechanical defect, improper weight and balance, or other issues. He may not be able to safely guarantee the aircraft is trimmed in such conditions.
There is a related question here that explains it in further detail: What is an out-of-trim condition and how is it detected?
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4$\begingroup$ This can, however, only be detected when flying. On the ground, “out of trim” means balance out of limits. $\endgroup$ Commented Nov 9, 2018 at 20:38
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$\begingroup$ Trim controls speed. The airplane will seek, in the absence of a pilot elevator input, a given speed based on where the trim device, tab or moveable stabilizer, is set. If an airplane is pitching up as in your example,it means it's going faster than its trim speed and is trying to slow down to achieve equilibrium. When you adjust the trim nose down to stop a pitch movement, you are increasing the airplane's trim speed to match its current speed. $\endgroup$– John KCommented Nov 9, 2018 at 20:44
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If the center of gravity was out of limits it is unlikely the crew would use the term "trim" to describe it. Also, if passengers needed to be moved to correct the condition you would see this happening.
More likely the fuel control unit was out of trim and needed to be adjusted. It is a fairly common procedure if an engine parameter is a little bit off from normal.
From Flight-Mechanic.com:
The field repair of the turbine engine fuel control is very limited. The only repairs permitted in the field are the replacement of the control and adjustments afterwards. These adjustments are limited to the idle rpm and the maximum speed adjustment, commonly called trimming the engine. Both adjustments are made in the normal range of operation. During engine trimming, the fuel control is checked for idle rpm, maximum rpm, acceleration, and deceleration. The procedures used to check the fuel control vary depending on the aircraft and engine installation.
The engine is trimmed in accordance with the procedures in the maintenance or overhaul manual for a particular engine. In general, the procedure consists of obtaining the ambient air temperature and the field barometric pressure (not sea level) immediately preceding the trimming of the engine. Care must be taken to obtain a true temperature reading comparable to that of the air that enters the engine. Using these readings, the desired turbine discharge pressure or EPR (engine pressure ratio) reading is computed from charts published in the maintenance manual.
answered Nov 13, 2018 at 21:41
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1$\begingroup$ Interesting. I know little about jet engines so you may be right. $\endgroup$ Commented Nov 14, 2018 at 13:26
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1$\begingroup$ It would be ironic if my top-voted answer ever (and only a few days old!) were wrong, while all my answers about the relationship between thrust, lift, and climb angle, which are correct, are showing an average vote score around zero. I don't think it's really true that the best answers rise to the top at Aviation SE! $\endgroup$ Commented Nov 14, 2018 at 13:34
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1$\begingroup$ Ironic indeed! It is a well crafted answer though. Hard to know the credentials of those answering and voting here... $\endgroup$ Commented Nov 14, 2018 at 17:00