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shouldbworking

4,015 posts

97 months

[news] 
Thursday 4th October 2012 quote quote all
Must have been particularly unlucky I imagine - to know you are lost and report in as such, you'd assume they would have been very concerned over a possible CFIT, yet thats exactly what happened.

Would a vintage a/c like this have an artificial horizon?

Eric Mc

76,418 posts

150 months

[news] 
Thursday 4th October 2012 quote quote all
Here's a DH84 instrument panel. It looks like there is an artficial horizon in the left of centre of the panel -




Older artificial horizons had all black faces with a white line indicating the horizon as opposed to the more modern types which are divided between blue for sky and black for ground.





Edited by Eric Mc on Thursday 4th October 15:27

Burnedout

478 posts

75 months

[news] 
Friday 5th October 2012 quote quote all
Eric Mc said:
Very, very sad.

Should not have been flying that day, I suppose.
Wasn't there, however, I reckon he would have known the front was coming, but thought he could get through before it come over. He didn't make it as it came through faster than expected & the sou'easter wasn't expected to hit as hard as it did.

Simpo Two

59,557 posts

150 months

[news] 
Friday 5th October 2012 quote quote all
Perhaps he was under pressure from the passengers to push on? Such things do cause error of judgement. He could have flown away from the storm to safety. You only have to gamble and lose once, sadly.

Mojocvh

14,845 posts

147 months

[news] 
Sunday 7th October 2012 quote quote all
Altimeter down bottom right.

Kinda points to a VMC type of aircraft/operational design regemé.

Hell, not even the old "T" instrument panel layout, as nothing more than an ex spacer that would put the weebie geebies up me straight away....

What a tragedy for all concerned, heartfelt condolences, MoJo.
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Ginetta G15 Girl

1,749 posts

69 months

[news] 
Monday 8th October 2012 quote quote all
Don't forget the DH84 first flew in 1932 and entered commercial service in 1933, so somewhat before the RAF's development of the 'Standard T' for the instrument layout.

Yes the 'Scan' would be a bit of a pain for IF work, but it's perfectly do-able. So to say it's a VMC only cockpit is a bit disingenuous IMO.

Having said that, early A/Hs as depicted there were/are notorious for erecting to false datums if you are not really careful with what you are doing, let alone the fact that they will gimbal and topple in extreme attitudes.

With that set up I would say you'd need to be pretty good at Limited Panel Instrument Flying to be in any way safe IMC. Unfortunately, IME most Civie pilots are not that well trained in this respect.



Edited by Ginetta G15 Girl on Monday 8th October 23:19

Simpo Two

59,557 posts

150 months

[news] 
Tuesday 9th October 2012 quote quote all
Ginetta G15 Girl said:
early A/Hs as depicted there were/are notorious for erecting to false datums
Reacting?

Ginetta G15 Girl

1,749 posts

69 months

[news] 
Tuesday 9th October 2012 quote quote all
No, erecting.

An A/H is a gyro driven instrument with the gyro mounted in a sealed housing and spinning in a horizontal plane about the vertical axis (this axis being relative to the ground). The case is gimbaled to allow rotation about the pitch and roll axes.

When the gyro is spun up, the nature of 'Gyroscopic Rigidity' means that the Horizon Bar becomes fixed in space and parallel to the natural horizon; thus movements of the a/c, and hence the movements of the instrument case with respect to the gyro, are shown on the face of the instrument as pitch and bank attitude changes of the aircraft symbol with respect to the horizon bar. In early Instruments such as here, 'spin up' is achieved via porting air against drive vanes on either side of the gyro rotor before sucking it out out via 4 equally spaced exhaust ports.

Note I said that the gyro must be spinning in a horizontal plane about a vertical axis.

It is essential that this vertical axis (datum) is correctly maintained for the instrument to continue to present accurate information WRT the natural horizon. This vertical axis is maintained via precession.

In this Instrument the 4 exhaust ports are each half-covered by a 'pendulous vane'. These allow equal discharge of air through each port when the gyro rotor is properly erected. Thus, if the rotor tilts for any reason, there will be a disturbance in the balance of the pendulous vanes. The result is that one vane of an opposite pair will tend to close, while its partner opens by a corresponding amount. The increase in air flow through the more open (ie less closed) port thus exerts a precessing force on the rotor housing, thereby re-erecting the gyro. Once the gyro has re-erected, the pendulous vanes return to a balanced condition.

Thus, anything that causes the gyro rotor to erect to a false datum must result in erroneous information being displayed to the pilot.

Such errors will include anything that prevents the suction system from operating properly, as well as things like friction in the system or, indeed, wear.


However there is another group of errors that this instrument will suffer from deriving directly from its design and the principles of the way it operates:


1. Incorrect pitch and bank indications will occur during normal coordinated (balanced) turns. These errors are the result of the pendulous vanes reacting to centripetal force, causing precession of the gyro toward the inside of the turn. After roll-out the instrument will show a slight climb and turn in the opposite direction. Generally speaking the error is greatest in a 180 degree steep turn - in a 360 degree turn the precession in the first half of the turn is cancelled out by that in the latter half. Also, generally, the precession error is around 3 to 5 degrees and is should be fairly rapidly cancelled out via pendulous vane action once straight and level flight is re-achieved. However, owing to the nature of the erection system, a balanced turn of less than 5 to 7 degrees of bank will cause the pendulous vane system to erect the gyro to a false vertical datum erroneously indicating the a/c to be in a shallow turn the other way upon roll-out and this error takes far longer for the pendulous vane system to correct.

2. Owing to inertia, a skidding turn will cause the pendulous vanes to move from their vertical position, thus precessing the gyro toward the inside of the turn. After the aircraft returns to straight-and-level flight, the instrument will indicate the a/c to be in a turn in the direction opposite the skid. In a Slipping turn the opposite applies.

3. A/C acceleration or deceleration will also induce precession errors, the degree being depending upon the amount of force applied. During acceleration the horizon bar will move down, thus indicating a climb. If one then applies pitch to correct this indication you will actually be selecting a pitch attitude lower than that shown by the Instrument (ie possibly a descent will be selected for). The opposite will apply in cases of deceleration.


As you can see, there are any number of pitfalls for the unwary or the inexperienced Instrument Pilot when using this system, and the last place you would want to find this out is when IMC!

Simpo Two

59,557 posts

150 months

[news] 
Tuesday 9th October 2012 quote quote all
OK I'm impressed!

Now, given that this chap was heading towards a honking great cloudbank/storm, and seemed to know he wasn't safe in cloud, why did he not turn round? Or was the position such that there was no escape whatever he did?

Mojocvh

14,845 posts

147 months

[news] 
Tuesday 9th October 2012 quote quote all
Ginetta G15 Girl said:
No, erecting.

An A/H is a gyro driven instrument with the gyro mounted in a sealed housing and spinning in a horizontal plane about the vertical axis (this axis being relative to the ground). The case is gimbaled to allow rotation about the pitch and roll axes.

When the gyro is spun up, the nature of 'Gyroscopic Rigidity' means that the Horizon Bar becomes fixed in space and parallel to the natural horizon; thus movements of the a/c, and hence the movements of the instrument case with respect to the gyro, are shown on the face of the instrument as pitch and bank attitude changes of the aircraft symbol with respect to the horizon bar. In early Instruments such as here, 'spin up' is achieved via porting air against drive vanes on either side of the gyro rotor before sucking it out out via 4 equally spaced exhaust ports.

Note I said that the gyro must be spinning in a horizontal plane about a vertical axis.

It is essential that this vertical axis (datum) is correctly maintained for the instrument to continue to present accurate information WRT the natural horizon. This vertical axis is maintained via precession.

In this Instrument the 4 exhaust ports are each half-covered by a 'pendulous vane'. These allow equal discharge of air through each port when the gyro rotor is properly erected. Thus, if the rotor tilts for any reason, there will be a disturbance in the balance of the pendulous vanes. The result is that one vane of an opposite pair will tend to close, while its partner opens by a corresponding amount. The increase in air flow through the more open (ie less closed) port thus exerts a precessing force on the rotor housing, thereby re-erecting the gyro. Once the gyro has re-erected, the pendulous vanes return to a balanced condition.

Thus, anything that causes the gyro rotor to erect to a false datum must result in erroneous information being displayed to the pilot.

Such errors will include anything that prevents the suction system from operating properly, as well as things like friction in the system or, indeed, wear.


However there is another group of errors that this instrument will suffer from deriving directly from its design and the principles of the way it operates:


1. Incorrect pitch and bank indications will occur during normal coordinated (balanced) turns. These errors are the result of the pendulous vanes reacting to centripetal force, causing precession of the gyro toward the inside of the turn. After roll-out the instrument will show a slight climb and turn in the opposite direction. Generally speaking the error is greatest in a 180 degree steep turn - in a 360 degree turn the precession in the first half of the turn is cancelled out by that in the latter half. Also, generally, the precession error is around 3 to 5 degrees and is should be fairly rapidly cancelled out via pendulous vane action once straight and level flight is re-achieved. However, owing to the nature of the erection system, a balanced turn of less than 5 to 7 degrees of bank will cause the pendulous vane system to erect the gyro to a false vertical datum erroneously indicating the a/c to be in a shallow turn the other way upon roll-out and this error takes far longer for the pendulous vane system to correct.

2. Owing to inertia, a skidding turn will cause the pendulous vanes to move from their vertical position, thus precessing the gyro toward the inside of the turn. After the aircraft returns to straight-and-level flight, the instrument will indicate the a/c to be in a turn in the direction opposite the skid. In a Slipping turn the opposite applies.

3. A/C acceleration or deceleration will also induce precession errors, the degree being depending upon the amount of force applied. During acceleration the horizon bar will move down, thus indicating a climb. If one then applies pitch to correct this indication you will actually be selecting a pitch attitude lower than that shown by the Instrument (ie possibly a descent will be selected for). The opposite will apply in cases of deceleration.


As you can see, there are any number of pitfalls for the unwary or the inexperienced Instrument Pilot when using this system, and the last place you would want to find this out is when IMC!
Pretty good! [says one of the last electrical and gen inst fitters!]

RichB

30,120 posts

169 months

[news] 
Tuesday 9th October 2012 quote quote all
Simpo Two said:
OK I'm impressed!
http://www.pilotfriend.com/training/flight_training/fxd_wing/attitude.htm
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