Question - Attitude Indicator
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Question - Attitude Indicator
Looking for opinions, teachings, and learned behaviours.
Should the attitude indicator be adjusted for instrument flight after departure. For instance - to show the bar level with horizon in cruise flight, or just leave it alone ?
Trying to see it from both sides...Thanks all.
Should the attitude indicator be adjusted for instrument flight after departure. For instance - to show the bar level with horizon in cruise flight, or just leave it alone ?
Trying to see it from both sides...Thanks all.
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As with any aviation question ; it depends.
If flying an aircraft like a S-76 that lands at 10 degrees nose up and the tail makes contact with the ground at 15 degrees then it is best to set the AI level on the runway and leave it there, if your flying a Bell it probably dosen't matter.
If flying an aircraft like a S-76 that lands at 10 degrees nose up and the tail makes contact with the ground at 15 degrees then it is best to set the AI level on the runway and leave it there, if your flying a Bell it probably dosen't matter.
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Agree with IHL. While I can't speak for Bell products, I can say I've experimented with the ADI for flight in IMC in an S76. After many years of cloud time, I've determined (for myself anyway) the best thing to do is to leave it alone and accept what it shows you in flight.
It's only my opionion, but if you re-adjust it in flight, you may forget to reset it later on. If it's a real low vis day and you're a bit tired, who knows, it may be the difference between having or not having an embarrassing moment at the end of the flight.
It's only my opionion, but if you re-adjust it in flight, you may forget to reset it later on. If it's a real low vis day and you're a bit tired, who knows, it may be the difference between having or not having an embarrassing moment at the end of the flight.
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Here is a situation that took place many years ago and it involved a group of US Army H-34s that were on training mission in Europe. I do not believe that any of the pilots were instrument rated. They were in a trailing formation flying in a valley when the two lead helicopters ran into a fog bank. The flight leader contacted the trailing members telling them to come to a hover and the last one in line would reverse course and fly out of the valley followed by the next and so on. Meanwhile the leader and the man behind him were totally enveloped with no visual clues as to where they were relative to the ground or the sides of the valley. In a hover, the H-34 (as well as most Sikorsky helicopters) will be tail low by three degrees and left wheel down by seven degrees (explanation to follow). The attitude indicator indicated that the helicopter was in a slight climb and in a bank to the left. The airspeed indicated zero and the VSI indicated no climb or dive. The Pilots, it was assumed turned on their pitot heat but there was no change and with this they lost spatial orientation and crashed.
The reason the most Sikorsky helicopters hang tail low in a hover is that the transmission is tilted forward by three degrees and the head will align itself with the local horizon. The reason they hang left wheel down by seven degrees or thereabouts is that the mixing unit has a built in bias to compensate for tail rotor translation tilting the blade disc seven degrees to the left. The interlock from the blades will cause the helicopter to tilt as well.
Any comments Nick?
The reason the most Sikorsky helicopters hang tail low in a hover is that the transmission is tilted forward by three degrees and the head will align itself with the local horizon. The reason they hang left wheel down by seven degrees or thereabouts is that the mixing unit has a built in bias to compensate for tail rotor translation tilting the blade disc seven degrees to the left. The interlock from the blades will cause the helicopter to tilt as well.
Any comments Nick?
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To: Joe Pilot
Exactly why am I out to lunch?
This might help to explain what I stated above regarding the H-34s.
On the US Navy version of the H-34 (HSS-1) they incorporated an Automatic Stabilization System which in actuality was a slightly modified F-86 autopilot. During the installation of the ASE in the production cycle they would level the helicopter and they would place the black box on a jig that was mounted to the ASE mounts and this jig simulated the 7-degree tilt to the left and the 3-degree tail down position. They would then null out any error signal from the ASE and then this represented the normal attitude of the helicopter. So when the helicopter hovered there was no error signal even though the helicopter was not level with the horizon. They even incorporated this capability on the Sonar Dipping System. When the helicopter went into a hover and dropped the sonar ball the cable was contained between two pick off devices that indicated the sonar ball was not normal to the helicopter and this generated an error signal to the ASE, which would move the helicopter. Once the ball was lowered and the error signal generated the operator would null out the signals and the ball would hang straight down from the helicopter, which was tipped to the left and aft.
Now, the pilots of the H-34s didn’t have an ASE so they had to rely on the instruments and without instrument training they ended up crashing because they were trying to correct for the indication on the Attitude Indicator. This is all documented in US Army records.
The Army eventually added the ASE to some if not all H-34s.
[ 04 December 2001: Message edited by: Lu Zuckerman ]
[ 04 December 2001: Message edited by: Lu Zuckerman ]
Exactly why am I out to lunch?
This might help to explain what I stated above regarding the H-34s.
On the US Navy version of the H-34 (HSS-1) they incorporated an Automatic Stabilization System which in actuality was a slightly modified F-86 autopilot. During the installation of the ASE in the production cycle they would level the helicopter and they would place the black box on a jig that was mounted to the ASE mounts and this jig simulated the 7-degree tilt to the left and the 3-degree tail down position. They would then null out any error signal from the ASE and then this represented the normal attitude of the helicopter. So when the helicopter hovered there was no error signal even though the helicopter was not level with the horizon. They even incorporated this capability on the Sonar Dipping System. When the helicopter went into a hover and dropped the sonar ball the cable was contained between two pick off devices that indicated the sonar ball was not normal to the helicopter and this generated an error signal to the ASE, which would move the helicopter. Once the ball was lowered and the error signal generated the operator would null out the signals and the ball would hang straight down from the helicopter, which was tipped to the left and aft.
Now, the pilots of the H-34s didn’t have an ASE so they had to rely on the instruments and without instrument training they ended up crashing because they were trying to correct for the indication on the Attitude Indicator. This is all documented in US Army records.
The Army eventually added the ASE to some if not all H-34s.
[ 04 December 2001: Message edited by: Lu Zuckerman ]
[ 04 December 2001: Message edited by: Lu Zuckerman ]
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IMHO,the following system works best. I always set the AI at the IAS that I intend to use if either:
a. Planned IFR transit - Cruise speed.
b. VFR, but with a possibility of an IFR abort. Approach or comfortable IMC manoeuvring speed.
It's quite simple to set it on departure pre entry into IMC or at a known datum, later refined at the desired IAS to give s&L flight at the appropriate power setting.
This is obviously dependant on a/c type as some have marked differences in pitch attitude in cruise and approach speeds.
dm - I don't think that accepting whatever it shows in flight is likely to be of much assistance as it may well give you an attitude markedly different from the optimum IFR speed. Again this is type dependant.
Lu - I think you've missed the point, I can't think of any helicopter pilot who would willingly try to hover IMC, unless it was for the most compelling of reasons and with the benefit of a full hands off autopilot hover facility. There ain't many of them about!!
a. Planned IFR transit - Cruise speed.
b. VFR, but with a possibility of an IFR abort. Approach or comfortable IMC manoeuvring speed.
It's quite simple to set it on departure pre entry into IMC or at a known datum, later refined at the desired IAS to give s&L flight at the appropriate power setting.
This is obviously dependant on a/c type as some have marked differences in pitch attitude in cruise and approach speeds.
dm - I don't think that accepting whatever it shows in flight is likely to be of much assistance as it may well give you an attitude markedly different from the optimum IFR speed. Again this is type dependant.
Lu - I think you've missed the point, I can't think of any helicopter pilot who would willingly try to hover IMC, unless it was for the most compelling of reasons and with the benefit of a full hands off autopilot hover facility. There ain't many of them about!!
My advice for what it is worth?
Erect it on the ground ( the artificial horizon that is) and then leave it well alone. It doesn't matter if it shows a couple of degrees nose up or down in flight as it is only showing a relative attitude to the true horizon. One has to include RoC@D indicator, altimeter, ASI, slip indicator and compass in ones scan to get the total picture.
As to the formation of a/c coming to the hover IMC and then reversing out - this sounds a very tall tale.
[ 04 December 2001: Message edited by: roundwego ]
Erect it on the ground ( the artificial horizon that is) and then leave it well alone. It doesn't matter if it shows a couple of degrees nose up or down in flight as it is only showing a relative attitude to the true horizon. One has to include RoC@D indicator, altimeter, ASI, slip indicator and compass in ones scan to get the total picture.
As to the formation of a/c coming to the hover IMC and then reversing out - this sounds a very tall tale.
[ 04 December 2001: Message edited by: roundwego ]
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To: Reg C Elley and Roundwego
The two pilots that found themselves in the fog had absolutely no idea how far they were from the walls of the canyon. This was I believe was a compelling reason to hover. Why they didn’t just pull collective and rise vertically above the fog I don’t know and obviously it didn’t cross the minds of the two pilots. The other pilots were in VFR conditions and they knew were they were relative to the canyon walls. I believe if these pilot were instrument rated they would have enough presence of mind to go up. These pilots my have been junior warrant officers with minimum time in this helicopter let alone any other helicopter I can’t say. But as I indicated previously it is recorded in Army records.
Regarding the ASE installation on H-34s this didn't happen until much later so the pilots were controlling by hand.
[ 05 December 2001: Message edited by: Lu Zuckerman ]
The two pilots that found themselves in the fog had absolutely no idea how far they were from the walls of the canyon. This was I believe was a compelling reason to hover. Why they didn’t just pull collective and rise vertically above the fog I don’t know and obviously it didn’t cross the minds of the two pilots. The other pilots were in VFR conditions and they knew were they were relative to the canyon walls. I believe if these pilot were instrument rated they would have enough presence of mind to go up. These pilots my have been junior warrant officers with minimum time in this helicopter let alone any other helicopter I can’t say. But as I indicated previously it is recorded in Army records.
Regarding the ASE installation on H-34s this didn't happen until much later so the pilots were controlling by hand.
[ 05 December 2001: Message edited by: Lu Zuckerman ]
Depends what kind of an AI you've got as to the nitty gritty details, but in my experience they all have some kind of rate-sensitive auto erection system (oo er!) that will, in relatively unaccelerated flight, slowly return the indicator to a gravity-sensed horizon. Therefore if you erect it to some other random horizon in the hover or cruise, it will simply wander back to a true-ish vertical after a while.
Also, if you erect an AI gyro to a false horizon, you will get what they call a 'pitch-roll' error, whereby the indication will vary depending on which way you're heading. Try it and see one day:
Put on 20 degrees of bank, for example, and erect the AI to that false horizon, and roll out on a heading of, say, East.
Then, when you turn onto North or South, your AI will go wings level, but will indicate 20 degrees nose up or down, depending on which way you turned. It all has to do with the gyro's rigidity in space - just imagine the gyro spinning away rigidly and the aircraft and gimbals rotating around it as your attitude and heading changes.
So basically, you might as well set it to 0 pitch wings level when you're in a level attitude (visually in flight, or on the ground or whatever) and then just remember the attitudes needed for given phases of flight with reference to that.
Hope that is of some help.
Also, if you erect an AI gyro to a false horizon, you will get what they call a 'pitch-roll' error, whereby the indication will vary depending on which way you're heading. Try it and see one day:
Put on 20 degrees of bank, for example, and erect the AI to that false horizon, and roll out on a heading of, say, East.
Then, when you turn onto North or South, your AI will go wings level, but will indicate 20 degrees nose up or down, depending on which way you turned. It all has to do with the gyro's rigidity in space - just imagine the gyro spinning away rigidly and the aircraft and gimbals rotating around it as your attitude and heading changes.
So basically, you might as well set it to 0 pitch wings level when you're in a level attitude (visually in flight, or on the ground or whatever) and then just remember the attitudes needed for given phases of flight with reference to that.
Hope that is of some help.
Invisible 1,
Getting back to your question, IMHO you should set the AH on the ground, adjust the bar, and leave it alone for the rest of the flight. Any thought of caging/resetting the gyro in flight is fraught with problems: as long as the AH was wings level on the ground, the nose up/down attitude will be reflected accurately throughout flight.
Taken to extremes, a two pilot crew who have set their AH horizons to a different datum could create a potential hazard should there be an unflagged instrument failure, whereas a crosscheck to AH's set to a common datum would be far more easily compared to each other, and the standby horizon.
Some helicopters cruise markedly tail down (A109, for instance), and you soon get used to seeing 4-5 degrees nose up in cruise. Your IF skills, as previously mentioned, will complete a full scan of other inputs to tell you what the machine is actually doing.
Getting back to your question, IMHO you should set the AH on the ground, adjust the bar, and leave it alone for the rest of the flight. Any thought of caging/resetting the gyro in flight is fraught with problems: as long as the AH was wings level on the ground, the nose up/down attitude will be reflected accurately throughout flight.
Taken to extremes, a two pilot crew who have set their AH horizons to a different datum could create a potential hazard should there be an unflagged instrument failure, whereas a crosscheck to AH's set to a common datum would be far more easily compared to each other, and the standby horizon.
Some helicopters cruise markedly tail down (A109, for instance), and you soon get used to seeing 4-5 degrees nose up in cruise. Your IF skills, as previously mentioned, will complete a full scan of other inputs to tell you what the machine is actually doing.
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In the Bells we were setting the Attitude Indicator at 5 degrees on the ground so that it would read wings level in cruise flight. You can also set the AI wings level in flight by checking the VSI and altimeter. Lu, the Army aviators should have briefed on inadvertant IMC prior to flight.
[ 05 December 2001: Message edited by: GulfPLt ]
[ 05 December 2001: Message edited by: GulfPLt ]
I think a quick distinction of terms may help: I understand "cage" to mean erect the AH/AI in relation to a provided reference (usually the aircraft's attitude at the time of cage). As Arm OTW said, most AI's will then try to slowly erect to the real verticle anyway.
I understand "set" the AH/AI to mean setting the pitch bar and sky pointer (roll indicator) to what ever you desire.
In the UH-60 we left it alone. In the UH-1H we ensured the sky pointer was good before lift off, then set 5 degrees nose up in an in to wind hover prior to take off. I believe "caging" the indicator at any time other than on the ground is frought with complications. Also, as the CofG can change considerably in flight (especially in UH-1 type rotor heads) you simply adopted an attitude that worked, and adjusted it according to CofG during the flight. Continually setting the indicator resulted in a lot of inaccurate flying. My two cents.
Lu,
The average time in cloud for a non instrument rated pilot before becomming spatially disorientated was less than 125 seconds. The average time to hit the ground and by the farm was 178 seconds.
This was from fixed wing at altitudes below 10,000. My guess is trying to hover (as opposed to doing inadvertant IMC drill) at tree tops with no visual reference and with no auto hover system - the average time to spear in would be 20 secs or less, with or without an instrument rated pilot at the controls. I reckon if you lasted any longer than a minute you would be a guru (or BS artist).
I understand "set" the AH/AI to mean setting the pitch bar and sky pointer (roll indicator) to what ever you desire.
In the UH-60 we left it alone. In the UH-1H we ensured the sky pointer was good before lift off, then set 5 degrees nose up in an in to wind hover prior to take off. I believe "caging" the indicator at any time other than on the ground is frought with complications. Also, as the CofG can change considerably in flight (especially in UH-1 type rotor heads) you simply adopted an attitude that worked, and adjusted it according to CofG during the flight. Continually setting the indicator resulted in a lot of inaccurate flying. My two cents.
Lu,
The average time in cloud for a non instrument rated pilot before becomming spatially disorientated was less than 125 seconds. The average time to hit the ground and by the farm was 178 seconds.
This was from fixed wing at altitudes below 10,000. My guess is trying to hover (as opposed to doing inadvertant IMC drill) at tree tops with no visual reference and with no auto hover system - the average time to spear in would be 20 secs or less, with or without an instrument rated pilot at the controls. I reckon if you lasted any longer than a minute you would be a guru (or BS artist).
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Reg C Elley says:
"I don't think that accepting whatever it shows in flight is likely to be of much assistance as it may well give you an attitude markedly different from the optimum IFR speed. Again this is type dependant."
Perhaps I should expand a bit as to my intended meaning. In my case, for example, some of the older 76s I've flown had ADIs with an adjustable sphere inside. Auto-levelling features notwithstanding, I could trim to an airspeed then reset my horizon to indicate level at that airspeed.
In the EFIS equipped aircraft I am now flying, this is not an option. What the ADI shows me is what I have to work with, if this means straight and level flight but with 2 or 3 degrees nose down, then so be it. I have other instruments to help determine if I am truly level.
If resetting the ADI once in cruise makes one feel better that's fine, it still won't change the actual attitude of the aircraft and may well lead to a false sense of fuselage postion.
"I don't think that accepting whatever it shows in flight is likely to be of much assistance as it may well give you an attitude markedly different from the optimum IFR speed. Again this is type dependant."
Perhaps I should expand a bit as to my intended meaning. In my case, for example, some of the older 76s I've flown had ADIs with an adjustable sphere inside. Auto-levelling features notwithstanding, I could trim to an airspeed then reset my horizon to indicate level at that airspeed.
In the EFIS equipped aircraft I am now flying, this is not an option. What the ADI shows me is what I have to work with, if this means straight and level flight but with 2 or 3 degrees nose down, then so be it. I have other instruments to help determine if I am truly level.
If resetting the ADI once in cruise makes one feel better that's fine, it still won't change the actual attitude of the aircraft and may well lead to a false sense of fuselage postion.
Typically the 76 has about a 3 degree nose down attitude on the AH. That figures well to what the attitude of the aircraft is in fwd flight.
I consider it foolish to mess with the AH in flight. Set it on the ground while the aircraft is laterally level and leave it be. Be wary that while flying oleo equipped machines that unequal compression of the oleos will create an error.
On the EFIS helos the AH was constantly leaning. You eventually learned to crosscheck with your Standby and have a glance at the other sides attitude to help diagnose the problem.
I consider it foolish to mess with the AH in flight. Set it on the ground while the aircraft is laterally level and leave it be. Be wary that while flying oleo equipped machines that unequal compression of the oleos will create an error.
On the EFIS helos the AH was constantly leaning. You eventually learned to crosscheck with your Standby and have a glance at the other sides attitude to help diagnose the problem.
