Pitch trim - poorly understood?
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Seriously Otto?,
Boeing could not make it clearer by including such fact in the QRH under "windshear escape maneuver" AUTOMATIC.
Obviously as a pilot you would know that 1AP ON+TOGA=AP OFF and 2 AP ON +TOGA=AP ON.
Derfred,
Refreshing,thank you.
Boeing could not make it clearer by including such fact in the QRH under "windshear escape maneuver" AUTOMATIC.
Obviously as a pilot you would know that 1AP ON+TOGA=AP OFF and 2 AP ON +TOGA=AP ON.
Derfred,
forward elevator happened in a go-around, I would simply reduce thrust (after all, the aircraft was in-trim at lower thrust just prior to the go-around). I certainly wouldn't be flinging it into a 60 degree bank at low level.
I think that although there are in-extremis recovery techniques for most sort of LoC scenarios, the very fact that the departure from the normal flight regime has happened while the pilot was on the controls and maybe because they were on the controls, it means that they are unlikely to be able to take much appropriate action in the near future as they have basically lost SA. Near the ground that’s often fatal.
It’s been a while since I was on the 737 but I do remember doing a lot of GAs during sim and base training in various configurations. There were certainly issues with thrust and trim but not unsurmountable ones and after a few goes you got the general idea. The nastiest one was probably a CAT III auto approach with an engine failure on the GA: the aircraft was out-of-trim in all three axes and required some deft handling to get it back on an even keel...
It’s been a while since I was on the 737 but I do remember doing a lot of GAs during sim and base training in various configurations. There were certainly issues with thrust and trim but not unsurmountable ones and after a few goes you got the general idea. The nastiest one was probably a CAT III auto approach with an engine failure on the GA: the aircraft was out-of-trim in all three axes and required some deft handling to get it back on an even keel...
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Ok, so for aircraft with trimmable horizontal stabilisers (THS) you must never hold the trim switch engaged for extended periods, but must release the switch frequently, so as to allow the column to return to neutral - and allow the forces to diminish.
So, that process would be:
1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;
Then repeat. Blip, relax, assess. Blip, relax, assess.
But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.
But it seems that trimming a large, THS aircraft that way can be fatal.
Do pilots undergo retraining on how to apply trim when they transition onto these larger aircraft? It appears not.
Of course, when the trim change is small, pilots will instinctively employ and repeat the "blip, relax, assess". This is something they have learnt, by themselves, to do - as it is perfectly appropriate. But things are much more interesting when the trim change is large, it would appear.
Drawing on gonebutnotforgotten's excellent trimming article once again, I quote this passage from Boeing, relating to nose-high upsets:
Are pilots trained to know how much "some" nose-down trim is? And how to "avoid using too much trim"?
So, that process would be:
1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;
Then repeat. Blip, relax, assess. Blip, relax, assess.
But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.
But it seems that trimming a large, THS aircraft that way can be fatal.
Do pilots undergo retraining on how to apply trim when they transition onto these larger aircraft? It appears not.
Of course, when the trim change is small, pilots will instinctively employ and repeat the "blip, relax, assess". This is something they have learnt, by themselves, to do - as it is perfectly appropriate. But things are much more interesting when the trim change is large, it would appear.
Drawing on gonebutnotforgotten's excellent trimming article once again, I quote this passage from Boeing, relating to nose-high upsets:
... the use of some nose-down stabilizer trim should provide sufficient elevator control to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim.
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2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;
Then repeat. Blip, relax, assess. Blip, relax, assess.
3. Assess the remaining forces;
Then repeat. Blip, relax, assess. Blip, relax, assess.
Do pilots undergo retraining on how to apply trim when they transition onto these larger aircraft? It appears not.
Unable to reduce pitch with full forward movement? Trim forward until you can.
Far safer to follow the Boeing 737 FCTM advice on Upset Recovery that states: "If normal pitch control inputs do not stop an increasing nose attitude, rolling the airplane to a bank angle that starts the nose down should work. Bank angles of about 45 degrees, up to a maximum of 60 degrees, could be needed."
Nothing about trimming forward if unable to reduce pitch with full forward elevator
Centaurus, there is something about applying trim. The QRH tells you to " apply appropriate nose down stabiliser trim". There is an asterisk beside the statement that takes you to a warning note at the bottom of the page which tells you that excessive trim may lead to a loss of control.
The QRH trumps the FCTM.
I'm not saying it is well written or understood but to say that there is nothing about applying trim if unable to reduce attitude with elevator is incorrect and may mislead some readers.
The QRH trumps the FCTM.
I'm not saying it is well written or understood but to say that there is nothing about applying trim if unable to reduce attitude with elevator is incorrect and may mislead some readers.
Centaurus:
Further to my last post, I have just read the latest version of the FCTM which also recommends using trim if in a wings level nose high upset ( greater than 25 degrees nose up).
It states
What version number of the FCTM are you preaching from?
Further to my last post, I have just read the latest version of the FCTM which also recommends using trim if in a wings level nose high upset ( greater than 25 degrees nose up).
It states
In this situation the pilot should trade altitude for airspeed, and maneuver the airplane's flight path back toward the horizon. This is accomplished by the input of up to full nose-down elevator and the use of some nose-down stabilizer trim.
I think the previous paragraph needs a mention as well:
"In this situation the pilot should trade altitude for airspeed, and maneuver the airplane's flight path back toward the horizon. This is accomplished by the input of up to full nose-down elevator and the use of some nose-down stabilizer trim. These actions should provide sufficient elevator control power to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim. Pilots should not fly the airplane using stabilizer trim, and should stop trimming nose down when they feel the g force on the airplane lessen or the required elevator force lessen. This use of stabilizer trim may correct an out-of-trim airplane and solve a less-critical problem before the pilot must apply further recovery measures. Because a large nose-down pitch rate results in a condition of less than 1 g, at this point the pitch rate should be controlled by modifying control inputs to maintain between 0 g and 1 g. If altitude permits, flight tests have determined that an effective way to achieve a nose-down pitch rate is to reduce some thrust.”
Also, whacking on loads of bank near the ground I would regard as the ultimate, last-ditch, do-or-die manoeuvre, probably initiated because of a fault with the flying controls or some issue with the CofG.
If you are way out of trim on the stabiliser, at some point you’re going to have to trim it otherwise you’re going to spend the rest of the flight going around in small circles until the fuel runs out.
I think I stand by my statement. You need to regain elevator authority and as soon as you have it, i.e. you get the ability to reduce pitch back, you can stop trimming. At the point you realise that you can’t get the nose down are you in an upset or quickly approaching one? I would have said the latter, so decisive corrective action will get the attitude under control and thus the flightpath before any more extreme measures are necessary.
"In this situation the pilot should trade altitude for airspeed, and maneuver the airplane's flight path back toward the horizon. This is accomplished by the input of up to full nose-down elevator and the use of some nose-down stabilizer trim. These actions should provide sufficient elevator control power to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim. Pilots should not fly the airplane using stabilizer trim, and should stop trimming nose down when they feel the g force on the airplane lessen or the required elevator force lessen. This use of stabilizer trim may correct an out-of-trim airplane and solve a less-critical problem before the pilot must apply further recovery measures. Because a large nose-down pitch rate results in a condition of less than 1 g, at this point the pitch rate should be controlled by modifying control inputs to maintain between 0 g and 1 g. If altitude permits, flight tests have determined that an effective way to achieve a nose-down pitch rate is to reduce some thrust.”
Also, whacking on loads of bank near the ground I would regard as the ultimate, last-ditch, do-or-die manoeuvre, probably initiated because of a fault with the flying controls or some issue with the CofG.
If you are way out of trim on the stabiliser, at some point you’re going to have to trim it otherwise you’re going to spend the rest of the flight going around in small circles until the fuel runs out.
I think I stand by my statement. You need to regain elevator authority and as soon as you have it, i.e. you get the ability to reduce pitch back, you can stop trimming. At the point you realise that you can’t get the nose down are you in an upset or quickly approaching one? I would have said the latter, so decisive corrective action will get the attitude under control and thus the flightpath before any more extreme measures are necessary.
Last edited by FullWings; 6th Apr 2016 at 08:57. Reason: Early morning confusion...
As a secondary point to the above, I have always been slightly concerned about my airline's teaching that the moment you are in a go around (or indeed takeoff) both hands should be on the control column and away from the thrust levers. In a situation like this action in all controls is necessary and if your hands are not in the right place your life may be harder than necessary.
Coming from a manual thrust aircraft, the only time hands were off the thrust levers were between V1 and reducing to climb thrust. In many normal go arounds, action on the thrust levers is required reasonably promptly and having a hand there is an important tactile handling cue.
Coming from a manual thrust aircraft, the only time hands were off the thrust levers were between V1 and reducing to climb thrust. In many normal go arounds, action on the thrust levers is required reasonably promptly and having a hand there is an important tactile handling cue.
If you are way out of trim on the stabiliser, at some point you’re going to have to trim it otherwise you’re going to spend the rest of the flight going around in small circles until the fuel runs out.
If the nose of (say) a Boeing has been allowed to pitch up excessively (45 degrees or more) during a go-around because of delayed reaction by the pilot (poor instrument scan in IMC), then such will be the rapidity of airspeed reduction, that seconds may count to prevent the aircraft from stalling.
At slow speeds, elevator effectiveness is less. In addition, trimming the stab trim forward to a guesstimate appropriate position, takes a few seconds of time, keeping in mind the 737 QRH warning to: "Use pitch trim sparingly." The angle of change from 45 degrees or more nose up, to just below the horizon, takes a few valuable seconds but is needed to get airspeed increasing again.
Depending upon the rate and angle of pitch up, as well as pilot competency, there may not be time to wait and see if elevator and stab trim combined is working quickly enough before control is lost. The QRH continues: "Roll (adjust bank angle) to obtain a nose down pitch rate"
Clearly the pilot is racing against time to get the pitch down to safe manageable levels. Unloading and simultaneously rolling to get the nose dropping is more or less guaranteed to get the nose going down immediately. On the other hand, leaving those corrective actions too late by first waiting for full forward elevator and appropriate forward stab trim to take positive effect before resorting to a roll, might be the straw that broke the camel's back.
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Nothing about trimming forward if unable to reduce pitch with full forward elevator
I think some of the problem with discussions like these (and very useful they are too) is that we all have a scenario in the back of our minds when we post and it isn’t necessarily exactly the same as the ones others are considering.
For the sake of argument, let’s assume that:
a) The aircraft (737) is in normal working order
b) SOPs have been followed up to this point
c) A GA in landing config. is now initiated
Yes, the aircraft has a tendency to pitch up (not actually a bad thing in some circumstances) and this is enhanced on a coupled approach at low level. Is it uncontrollable? No. Would it become less controllable if pitch were allowed to increase to the point airspeed was being lost? Yes. If you still didn’t do anything would it carry on to the point of stalling as the elevator lost effectiveness? Possibly.
If a crew has got to this juncture without any action or intervention, then they are out of the loop. It’s too much to expect a timely recovery from a self-induced scenario: they were unable to diagnose a nose-high reducing-airspeed situation while it was happening, which often has a simple remedy, so what are the chances of correct recovery action being taken? Not good. We’re not talking about test pilots exploring the envelope, it’s line guys well out of their depth, for whatever reason (distraction, fatigue, gradient. etc.)
Possibly better to deal with the incipient low speed event with elevator and trim if required, maybe with an “I have control” in there somewhere. Unless something has actually broken then rolling to contain the pitch shouldn’t be necessary; you have it as a last resort but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations. Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?
For the sake of argument, let’s assume that:
a) The aircraft (737) is in normal working order
b) SOPs have been followed up to this point
c) A GA in landing config. is now initiated
Yes, the aircraft has a tendency to pitch up (not actually a bad thing in some circumstances) and this is enhanced on a coupled approach at low level. Is it uncontrollable? No. Would it become less controllable if pitch were allowed to increase to the point airspeed was being lost? Yes. If you still didn’t do anything would it carry on to the point of stalling as the elevator lost effectiveness? Possibly.
If a crew has got to this juncture without any action or intervention, then they are out of the loop. It’s too much to expect a timely recovery from a self-induced scenario: they were unable to diagnose a nose-high reducing-airspeed situation while it was happening, which often has a simple remedy, so what are the chances of correct recovery action being taken? Not good. We’re not talking about test pilots exploring the envelope, it’s line guys well out of their depth, for whatever reason (distraction, fatigue, gradient. etc.)
Possibly better to deal with the incipient low speed event with elevator and trim if required, maybe with an “I have control” in there somewhere. Unless something has actually broken then rolling to contain the pitch shouldn’t be necessary; you have it as a last resort but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations. Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?
but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations
One was a runaway propeller within seconds after liftoff in a Convair 440. The the other was a training flight in the circuit when we experienced a sudden full flap runaway to down while on three engines in a four engine aircraft. On both occasions things happened so fast that the copilots concerned had absolutely no idea what had happened until shortly after control was gained and I got my own breath back.
Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?
Last edited by Centaurus; 8th Apr 2016 at 02:29.
If a crew has got to this juncture without any action or intervention, then they are out of the loop. It’s too much to expect a timely recovery from a self-induced scenario: they were unable to diagnose a nose-high reducing-airspeed situation while it was happening, which often has a simple remedy, so what are the chances of correct recovery action being taken? Not good. We’re not talking about test pilots exploring the envelope, it’s line guys well out of their depth, for whatever reason (distraction, fatigue, gradient. etc.)
Possibly better to deal with the incipient low speed event with elevator and trim if required, maybe with an “I have control” in there somewhere. Unless something has actually broken then rolling to contain the pitch shouldn’t be necessary; you have it as a last resort but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations. Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?
Possibly better to deal with the incipient low speed event with elevator and trim if required, maybe with an “I have control” in there somewhere. Unless something has actually broken then rolling to contain the pitch shouldn’t be necessary; you have it as a last resort but it’s going to look awfully odd to the guy in the other seat unless you’re very quick with the explanations. Plus, if you have the skills and gonads to throw in 60degs of bank at low level why the *&%^ didn’t you do something less extreme earlier!?
Report following is remarkably similar to the FlyDubai 737crash sequence.
On 24 December 2015, the Interstate Aviation Committee released their final report stating that the crash was caused by an under-qualified crew who lacked the skills to recover from an excessive nose up attitude during a go-around procedure. The go-around was necessitated by a positional error in the navigation system, a map drift. The pilots’ deficiencies were caused by lack of airline safety management and lack of regulators’ oversight.[24][25]
According to the final report, during the final approach the crew initiated a go-around, but being under high workload, which possibly caused a "tunnel vision effect", they did not perceive warning messages related to auto-pilot disconnection. When the plane climbed to 700 m, its pitch angle reached 25 degrees and the airspeed dropped to 230 km/h.
At that moment the captain, who never performed a go-around before, apart from the training, moved the yoke, pitching nose down, which led to stopping climb and started descent and increase of the aircraft's airspeed. After reaching the altitude of 700 m, the aircraft started a steep nosedive, with the pitch angle reaching −75° when the aircraft impacted the ground. The plane crashed on the airport's runway with a speed exceeding 450 km/h. The time from the start of the go-around maneuver until the impact was about 45 seconds, including 20 seconds of aircraft descent
There have been some interesting observations in the gliding world, showing that a small proportion of people learning to fly are very sensitive to reduced or negative g and associate it with falling (stalling), giving an instinctive reaction to push their hands out in front of them to cushion the fall, so the theory goes. As they are holding the stick, this reduces the g even more, leading to more sensation, pushing, etc.
There have been quite a few unexplained dives into the ground in aircraft that have been found to have been serviceable pre-impact. I’ve been witness to two.
It is possible that someone could train as a commercial pilot without being exposed to this kind of scenario, especially in IMC. I’m not saying this is what happened in many of these crashes, just putting it forward for consideration...
There have been quite a few unexplained dives into the ground in aircraft that have been found to have been serviceable pre-impact. I’ve been witness to two.
It is possible that someone could train as a commercial pilot without being exposed to this kind of scenario, especially in IMC. I’m not saying this is what happened in many of these crashes, just putting it forward for consideration...
Far safer to follow the Boeing 737 FCTM advice on Upset Recovery that states: "If normal pitch control inputs do not stop an increasing nose attitude, rolling the airplane to a bank angle that starts the nose down should work. Bank angles of about 45 degrees, up to a maximum of 60 degrees, could be needed."
French investigators have revealed the first officer of a Falcon 7X used an escape technique from bombing exercises to regain control of the jet during a trim runaway event on 24 May 2011.
The aircraft trimmable horizontal stabiliser moved from its neutral position to 12 degrees down in 15 seconds as the 7X descended through 13,000 feet on approach to Kuala Lumpur's Subang Airport. Although the autopilot acted to counter the resulting pitch up, it disengaged after 8 seconds and the aircraft pitched up to 25 degrees.
The French speaking F/O who was flying realised his attempts to correct the pitch were ineffective but struggled to explain the situation to the English speaking captain.
He resorted to a bombing manoeuvre learned during his military career. He banked to 90 degrees to reduce pitch while the captain initially opposed the extreme 98 degree right bank. The F/O took priority control and maintained the bank at 40-80 degrees for 20 seconds.
This reflexive correction reduced the pitch from 41 degrees nose-up to 10 degrees and brought the angle of attack down to five degrees. The jet's airspeed fell to 150 knots from 297 knots during the upset which lasted 2 minutes and 30 seconds and subjected the aircraft to loads of up to 4.6g.
The BEA has credited the F/O for his rapid improvisation in recovering the Swiss registered aircraft (HB-JFN)"
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Quick thinking indeed and it showed the difficulty of trying to make a fast explanation to the other crew member of what's going on, when immediate action is the priority. Rolling into a steep turn to get the nose down from a rapidly increasing pitch angle seems not to be part of airline simulator training. The first officer's military training certainly paid off in this incident.
Last edited by Centaurus; 12th Apr 2016 at 03:02.
