Trimming below Vref?

Oakum_Boy

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I have an FO who trims continuously until touchdown. In order to get a smooth touchdown I guess. I've never felt it necessary to do this in any airplane. My question is that I don't think this is a wonderful technique, because if there were a need to balk the landing below Vref for say something on the runway, that may cause some problems when you give it TOGA thrust. Opening up this question to those with lots of experience in jets or anyone with an opinion? Thanks-
 

Lead Sled

Sitt'n on the throne...
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Trim, properly used, is a good thing. Proper technique in all planes that I'm aware of is to trim the airplane so that you could release the controls at anytime during climb, cruise, or descent and not have the airplane "wander off" on you. There is nothing more fatiguing than to have to hand fly a mistrimed or misrigged airplane for extended periods of time. (Autopilots don't like it much either.)

Like I've said before, you should probably avoid using the words "always" and "never" (or words like them) when it comes to aviation. For example, standard and acceptable technique when landing certain light taildraggers like the Aeronca Champ is to use full nose up trim on approach. But then again, you wouldn't have to worry much about "runaway trim" in a Champ and besides, where would it run to?


Pilots trim during the flare to relieve the back pressure that they are having to apply on the yoke. Remember from your flight training days that you trim for an airspeed. By adding nose up trim during the flare the pilots are, in essence, retrimming the airplane for a less than normal approach airspeed. As has been mentioned, in the event of a balked landing or go around, there's going to be a mad dash to get the airplane retrimmed and, in the mean time, the crew's going to need to put a lot of forward pressure on the yoke as the aircraft accelerates.

Is this a bad thing? It depends upon the characteristics of the airplane your happen to be flying and the conditions you're flying in. Solid IFR with an approach to minimums - probably not a good idea. VFR conditions, in an airplane where the stick forces aren't excessive, with landing assured and little or no chance of a go around - probably OK. But, just how difficult is it to leave the trim alone and add a few degrees of pitch?

Just a side note, if you're flying a jet and "working" the flare that much, the chances are you're going to be getting into an area of excessive float. A little excessive airspeed in the flare coupled with "working" the yoke to get a "greaser" and many airplanes have a tendency to get into ground effect and float and float and float. After that happens, the chance for a good touch down get really slim. In most jets, good technique is for your touchdown speed to be no less than Vref -10 knots.


'Sled






 

TonyC

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Oakum_Boy said:
I have an FO who trims continuously until touchdown.

... I don't think this is a wonderful technique, because ...
Have you discussed this with the FO? Does he even realize he's doing it? Has he given you a reason? (I'm guessing the answers are "No" sincec you also said, "In order to get a smooth touchdown I guess."

I'm guessing you could both benefit from such a discussion. Perhaps he knows of an advantage you haven't considered. Perhaps he hasn't considered a disadvantage that bothers you. Talk to him, and then share what you learn with us.

I don't usually think about trimming - - it's a reflex built into my thumb. When I have to exert smooth pressure on the yoke for more than a second or so, the thumb goes to work on the trim switch. Some T-37 Instructor did that to my thumb many years ago.


Speaking of T-37s... did any of you participate in those wagers concerning landings and touchdown speeds? You know, whoever touches down at the slower speed wins the coke? Wanna know the secret? Full nose-down trim. Yepp, full nose down trim puts the trim tab in a position to help the elevator, instead of relieving pressure on the stick. Yeah, it takes more back pressure, but the additional "elevator" control surface gives more control authority, ergo slower touchdown.

No Aeronautical engineering explanation to the contrary will be entertained here - - I have the cokes to prove it!


:)
 

A Squared

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Gratuitous Aeronautical discussion: (I'm only doing this because you told me not to, Tony)

That works on an airplane with elevator trim tabs (which I understand the T-37 has. On an airplane with a trimmable horizontal stabilzer, the reverse is true, trimming nose up will help you land at the lowest possible speed.

Also, on the DC-6, which has elevator trim tabs, trimming nose up will give you more elevator effectiveness in hte flare, because there is a stop which limits elevatore up[ travel, if you trim beyond (I think) 4 units ANU, the stop retracts and gives you 2 or 3 more degrees of elevator travel.
 
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