Artificial horizon Vs VSI instrument flying straight and level

[PelicanSquawk]

Hi All,

Please excuse my ignorance, with my question. Today I was practising some instrument flying with my instructor for the first time (pretty much). I was trying to hold her steady and level using the artificial horizon, and checking we weren't climbing or descending with the VSI and the altimeter. I am used to judging the attitude needed for straight and level flight by looking out of the cockpit and selecting the right attitude, double checking on the VSI. So my question is, would this attitude I select looking out of the cockpit typically equal "level wings" on the artificial horizon? Or are there instances (power settings) where the artificial horizon would show a slight nose up attitude for straight and level, and a sometimes level for a slight descent (with low power).

The reason I ask is because when I was trying to trim the aircraft for straight and level I think I was relying too much on the VSI and not quite trusting the AH, should I be looking for straight wings on the AH first and then confirming the 0 VSI reading, or is it the case that at many power settings the AH is in fact not level for straight level flight, and so I should be focussing more on the VSI?

I will talk to my instructor next time I see him, but just wanted to see what you guys recommend/think.

Thanks in advance for your help.

[B2N2]

Light aircraft rarely have an accurate enough attitude indicator.
It’s usually vacuum driven same as the DI and susceptible to precession also. Usually just sits there with a shimmy and a shake.
Altimeter and VS are better indicators.

[PelicanSquawk]

All the artificial horizon is doing is replicating the outside picture but just on a much smaller scale so think about flying visually...
High speed high power for straight and level is relatively low nose i.e. On the horizon. So what about slow flight? Less speed and less power you are going to require a more nose up attitude to maintain level flight so looking out the window you are going to see the nose sitting relatively higher to the horizon. This will be exactly mimicked on the artificial horizon. It is only showing you a scaled down version of what you see out the window and will not compensate etc to give you level flight by lining up the aircraft display onto the horizon.

On the 737 (only figures I know off the top of my head) it could be anywhere from 1-2 degrees nose up to 5-6 degrees nose up for level flight depending on speed and altitude. Similarly for descent on the approach we are typically nose slightly above the horizon while descending.

I am guessing you are still fairly early in training (PPL) and not onto formal IR training yet? This will all come in time and if that is correct you shouldn't really be doing too much flying on instruments as it is a specialised skill set to do accurately while not attracting all of your attention. Your instructor will be able to brief it better in person but for a light aircraft the VSI does have lag in it so I'd suggest initially setting the aircraft to a known attitude and make small corrections around that in the order of a degree or so. Your scan will be for example between aritificial horizon - altimeter (slight correction) - artificial horizon -altimeter (confirm correction worked or adjust) -- speed - artificial horizon - Direction Indicator - artificial horizon - altimeter repeat. The VSI in most light aircraft is just going to lag too much to be accurate enough for instrument flying as an initial level off but you certainly keep it in your scan for other areas of flight.

NB: You said towards the end about using the VSI to adjust attitude rather than the artificial horizon. If you are going to adjust the aircraft in pitch or roll or power you must be doing it by reference to the Artificial horizon and then cross check off the other primary instruments or you will get into a world of trouble my friend.


Thanks for the reply. This explanation was exactly what I was looking for, and makes a lot of sense. Yes I am fairly early on, we were flying like this today as the weather wasn't great, but I said I wouldn't mind giving the instrument flying a go. Just wanted to get some clarity on some thoughts I had after doing it.

Thanks again! Very Helpful.

[+TSRA]

The reason I ask is because when I was trying to trim the aircraft for straight and level I think I was relying too much on the VSI and not quite trusting the AH, should I be looking for straight wings on the AH first and then confirming the 0 VSI reading, or is it the case that at many power settings the AH is in fact not level for straight level flight, and so I should be focussing more on the VSI?

Focusing is the wrong method here. You should never, ever focus on one instrument for any single piece of information. This is what leads to pilots being surprised by a stall, spin, or spiral descent during the initial stages of instrument training.

The most useful phrase in any type of flying is the good old "power plus attitude equals performance" and remembering this will serve you well as you transition into instrument flying. Your VSI could indicate 0 fpm but, as TangoAlphad pointed out, your pitch attitude could be anywhere depending on the power setting.

Ask your instructor about the T-Scan. They should tell you that it starts by looking at the attitude indicator and moving about the instrument panel in a methodical method depending on what you're doing. You'll eventually find that this method is too tiring for anything more than a single training session and you will develop your own method in time, but the T-Scan does drive home the importance of combining sources of information to complete your mental picture.

For example, when climbing on a specific heading, the most important sources of information are pitch, airspeed, and heading. So you'll start the scan at the artificial horizon to set your pitch, look across to the airspeed indicator and ensure you're climbing at the required airspeed once you've set the power. Then look back to the artificial horizon to make sure you're still flying the correct pitch angle, before looking down to the directional gyro to ensure you're flying the proper heading. You'll then look back to the artificial horizon and start the process over.

After a couple scans, you'll include the altimeter and VSI in your scan, using the same method (e.g., AH - ALT - AH - VSI - AH). Every now and again you'll also include the engine instruments.

As you approach the target altitude, you'll include the altimeter more often in your scan than, say, the airspeed indicator. Once you're level, you'll readjust the pitch and power, and will rescan for what is now important during level flight (attitude, altitude, and heading).

Like I said, it is tiring moving your eyes, but the important thing is learning what information to look at when and, as you learn your aircraft and how it performs with various power and attitude settings, you'll learn to rely less and less on the T-Scan and more on your individual scan pattern.

Good luck!

[PelicanSquawk]

TSRA

Perfect, thanks. Great advice again, and I will put this to good use/bare it in mind for next time.

[Fl1ingfrog]

B2N2 what on earth are you on about.

PelicanSquawk we use a simple formula. It doesn't matter whether we are discussing a small or large aeroplane; i.e. a C150 or a Boeing 777; POWER + ATTITUDE = PERFORMANCE. It is also true that; power + performance = attitude and to complete the triangle attitude + performance = power.

The power is able to be set precisely to the required RPM (on most training aircraft). You know whether you have set it correctly because the RPM gauge tells you. The attitude is a different matter, it is a matter of judgement which must be taught and practiced. Having set the power and also the attitude on the horizon by judgement, the aircraft must stabilise. You can then check the achieved performance; straight and level at the required airspeed or climbing/descending at a given rate. Adjust as necessary.

The artificial horizon we better refer to as the attitude indicator (AI) because this more effectively describes its role. Some teaching suggests that you should refer to the AI to check your setting of the attitude on the natural horizon. In my view this incorrect for many reasons. The achieved performance best confirms whether you are correct or not as I have shown above.

The Altimeter indicates your altitude/height, which may be changing, this is all. The VSI is a trend instrument and in part is designed to lag. In certain circumstances it can indicate a descent/climb opposite to what is actually happening.

Finally; a horse comes before the cart not the other way round.

[scifi]

Nobody has yet mentioned the Turn Coordinator. Some crafty Instructors have the habit of pressing one of the rudder pedals. So all your efforts to keep the AI level, still results in an ever changing heading on the DI.
.

[+TSRA]

Nobody has yet mentioned the Turn Coordinator. Some crafty Instructors have the habit of pressing one of the rudder pedals. So all your efforts to keep the AI level, still results in an ever changing heading on the DI.

First, the Turn Coordinator was not mentioned because the OP was asking about altitude control, not directional control.

Second, that sort of behaviour by instructors in not professional and destroys student confidence. It’s certainly not crafty. A good instructor does not good induce a failure in the mind of the student, which this sort of “technique” does.

[rudestuff]

...Perhaps you’re reading a different question to the rest of us? I understand ‘straight and level’ to mean heading and altitude control.

[RatherBeFlying]

In most mechanical AH's there's a knob underneath that raises or lowers the little airplane to set a reference attitude.

[India Four Two]

It’s usually vacuum driven same as the DI and susceptible to precession also.

B2N2,

Can you expand on that? I was taught that for all practical purposes, the AH does not precess, because of the pendulous vanes.

[eckhard]

The air-driven artificial horizon is subject to acceleration and turning errors, which occur because of the precession of the acceleration forces applied to the pendulous vanes and their housing.

In a typical GA training aeroplane, the acceleration is insufficient to cause much of an error; on a fast-jet type, the error might be significant and indeed led to a few crashes on Meteors, Vampires, etc.

So, for all intents and purposes, in a GA training aircraft, the AH can be assumed to be error-free, apart from the self-induced possible error (mentioned by RatherBeFlying) of mis-setting the miniature airplane symbol.

Of course, the AH may topple if an extreme attitude is achieved, and in this case you are better off referring to the turn indicator and pressure instruments.

I would recommend the T-scan and other scans alluded to in previous posts. Although the AH can be regarded as the “master instrument”, the other ones play an important part in confirming whether or not a particular manoeuvre or performance target is being achieved.

I also endorse heartily the “Attitude plus power equals performance” mantra quoted by previous posters. I would add, that for more advanced I/F, you could add “configuration” to the left of the = sign.

Although I agree that the PPL syllabus should concentrate on visual flying, an understanding of the instruments and a working knowledge of some basic scan patterns will do no harm.

[+TSRA]

...Perhaps you’re reading a different question to the rest of us? I understand ‘straight and level’ to mean heading and altitude control.

Ordinarily, I would agree that a question relating to straight and level flight would include both altitude and directional control, in which case discussion including the Turn Coordinator and/or Directional Gyro would be applicable.

However, the OP was asking about pitch control as it relates to altitude - hence the expanded comments about the VSI and altimeter - during straight and level flight. Thus, one can assume that the directional component of "straight and level flight" is not a concern and can be ignored for simplicity.

[Cows getting bigger]

The VSI is the last instrument I would look at to judge S&L. It has so many inherent errors it's really only good for managing a rate of decent/climb.

AI should be the prime instrument for level flight with scan alternating between ASI and Alt. If you want the straight bit as well you add DI, which then brings Balance.

[Heston]

Personally I just look out the window. The natural horizon.

[galaxy flyer]

Personally I just look out the window. The natural horizon.

How does that work for you in cloud?


Gf

[Gertrude the Wombat]

So, for all intents and purposes, in a GA training aircraft, the AH can be assumed to be error-free ...
... apart from running out of vacuum ...

How does that work for you in cloud?
And then there's above cloud. Usually you still get a reasonable horizon, but one day the tops of the clouds were all over the place, some above me, and I was unable to keep straight and level by looking out of the window, despite being in bright sunshine. Someone else in the aircraft took over looking out and I went onto instruments.

[eckhard]

... apart from running out of vacuum ...

Good point! I would class that as a failure, rather than an error, but well worth bringing up.
A regular check of the suction gauge and a disciplined scan will help to catch that failure, although a ‘flag’ on the AH would be helpful. I’ve often wondered why that was not fitted.

[Fl1ingfrog]

Only the pilot knows he is in cloud, the aeroplane does not so the same rules apply; pwr + att continues to equal performance. If the aircraft is well trimmed and in balance the performance will remain constant. If the performance is constant the power and attitude are correct.

Gertrude the Wombat introduces an important human factors issue. Even in VMC we use our senses in a ratio of approximately 60% sight to 40% of other senses. So when we lose sight or, in this case, the sight information received by the brain is unusual, we become hyper sensitive to the other senses and so can soon become disorientated. The senses within the inner ear and "feelings" can become so strong that belief in the attitude instruments may be lost. The sensors in the inner ear become our primary source but erratic and continuous movements of the head will aggravate. Reliance therefore must be on the aircraft performance; speed, altitude and heading. If they are all good then leave the aeroplane alone. Any corrections if necessary must be small and never rushed.

[mikehallam]

12 volts run my EFIS AH, I assume it doesn't precess ?
mike hallam.

[Big Pistons Forever]

I think people over think instrument flying. Maintaining straight and level is exactly the same for IMC conditions as it is for VMC. The only difference is that instead of using the natural horizon in order to maintain the a wings level cruise pitch attitude you use the AI.

Like flying in VMC the majority of your time should be spent looking at the horizon, in this case the AI not the natural one. If the aircraft attitude is correctly maintained none of the other instruments are going to move much so job done.

Fl1ingfrog dos bring up a good point. For a new pilot you are your worst enemy as the airplane does not know it is in cloud. This is why getting some training in actual IMC conditions is very important in order to gain the confidence to believe your instruments.

[PelicanSquawk]

Thanks all! I have read all the replies, and concluded I was relying too much on the VSI, and not focussing enough on the AI. I flew again the next day in better conditions and made a point of comparing the attitude I set up by looking out of the cockpit with what I was seeing on the AI, so that I get used to how it should look. I also made a point to check the altimeter for indication if I was flying level, instead of the VSI.

Thanks again every one.

[tdbristol]

Maintaining straight and level is exactly the same for IMC conditions as it is for VMC.

On a more basic human factors viewpoint I wouldn't agree with that comment. In VMC what you see outside corresponds reasonably with what your body is feeling, whereas in IMC it can be very very different. Having had "the leans" in IMC I know that it can be hard to ignore the impulse to listen to what your body is telling you is 'clearly happening' and instead just believe the AI and other instruments. Many people who don't have IR currency and have entered IMC inadvertently have come off very badly 'knowing [by feel] that the plane is straight and level' when really it is in a descending turn, but have not believed or reacted enough to the AI and other instruments.

I would recommend that PelicanSquawk goes with an instructor in real IMC and tries various constant rate turns, climbs, descents etc. while at least the AI is obscured from your view to experience these effects.

[Fl1ingfrog]

tdbristol you are not correct. Gertrude the Wombat has given a very clear example of a loss of spacial awareness whilst in good VMC.

I would not recommend sole reference to instruments flying at this stage of his flying, it is far too early for this. He will be receiving the appropriate training later in his PPL course, as his instructor will best decide, when appropriate.

[PelicanSquawk]

Just to clarify, we did an instrument flight this early as the conditions were too poor for anything else. It was very much a “if you really want to go up we can do some instrument flying”. It was a very thought intensive 45 minutes, and I’ll probably hold off on flying in those conditions unless instrument flying is specifically what I should be working on.

[scifi]

The VSI is a most accurate instrument, and is very quick to show any alteration of altitude, just ask any Glider Pilot what they use to detect lift or sink.



I have a VSI and associated capacity flask, that I use for demonstration purposes. If I hold it near the ground, then raise it above my head, it will read +100 fpm, then slowly drop to zero. Lowering it again to floor level shows -100 fpm.



So if it accurately shows a height difference of 6 feet then that is good enough for me. An altimeter on the other hand would take one minute to change 100 ft, and it might not even do that, as they are well known for having some in-built friction.


The AI is also very insensitive, we are talking about fractions of the 3mm dot, to produce the same 100fpm change.
.

[lasseb]

I have a VSI and associated capacity flask, that I use for demonstration purposes. If I hold it near the ground, then raise it above my head, it will read +100 fpm, then slowly drop to zero. Lowering it again to floor level shows -100 fpm.

This sounds very interessting.

I wonder how fast you actually moved it? 100 fps is equiavalent to 0.5m/s i.e. approx 4 seconds from floor to over your head. My guess is that you are moving it much faster? Or is it not the instrument you are moving ?

What you probably have is an IVSI, that contains accelerometers which forses "false" pressure into the system when accelerated.
Try moving it slowly instead (like the 4 seconds from floor to top) and see if it actually displays 100 fpm.

The fact that is "slowly" drops to zero is the very lagging behaviour that causes som of the problems. If it where a precise instrument, it should show zero the moment you stop the movement.

[Fl1ingfrog]

The vario used in gliders is different in many respects than the simple VSI fitted in most light aircraft. The most common glider varios are compensated by also having a pitot source in addition to a static feed. This goes someway to eliminating sudden and short term changes to the indication which may also be caused by the pilot making pitch changes. A vario with an associated capacity flask is another method amongst others used. The gliding vario is also calibrated in knots rather than feet per minute.

The glider pilot seeks a constant upward rise in an air mass and similarly needs to be aware of a downward trend. Short term fluctuations are of little value to the glider pilot, but this is also true for powered pilots although for a different need. Because of it's limitations a simple VSI should be observed over a period of time to interpolate a trend. In light aircraft the altimeter is good enough to do this and fits within a normal instrument scan technique.

Power + attitude = performance in the first instance, not power + altimeter. Having set the power and attitude for S+L it is the stable airspeed that confirms you have done this correctly. Only when the speed has stabilised should the altimeter be assessed. Particularly important in the climb of course.

[RatherBeFlying]

You generally feel a thermal bump before the instrument shows it. I remember flying for an hour and a half with a dead vario.

With a working vario the needle is rarely steady in a thermal. The averager is your friend.

A spamcan VSI works off an orifice and will yield a rate over the last several seconds. It's major use is monitoring the descent rate on an ILS. Secondary is monitoring descent from cruise to be at a desired level at a fix for vectors or approach procedure. Until you are doing an instrument rating, a VSI doesn't offer much utility.

Edit: GtW has corrected me on glassy water landings.

Flying straight and level VFR, set your power and trim - and wait. Then see what the altimeter has done. It takes a while to get the knack.

Looking out for other traffic is the major priority. Very little time should be spent glancing at instruments. That's why glider varios have audio.

[Gertrude the Wombat]

Until you are doing an instrument rating, a VSI doesn't offer much utility.
Glassy water landings ...

[BEagle]

PelicanSquawk wrote: Just to clarify, we did an instrument flight this early as the conditions were too poor for anything else. It was very much a “if you really want to go up we can do some instrument flying”.

I guessed as much. Students should not be taught IF until they have completed solo circuit consolidation at the very earliest, otherwise they will develop bad 'head in' habits to the detriment of sound visual l00kout.

When I was a CFI, I banned my FIs from teaching IF too early; I found that some of them were airline wannabees and would fly 'demo IF' trips including their own ILS approaches (for which the student paid) which achieved nothing more than free practice for their own IRs until I put a stop to such negative and rather fraudulent activity.

[Duchess_Driver]

As has been said, but is often not explained often enough.... Power+Attitude=performance, but that is a vertical performance as well as a horizontal one.

WIth respect to the VSI, the majority of the time (and I think this is what CowsGettingBigger is alluding to) it is too sensitive to be part of the primary scan, often leads the student into chasing needles rather than developing an appropriate scan of the control and performance instruments.

[scifi]

Hi D.D... you are quite correct the VSI is a very sensitive instrument. The most likely reason is that it uses 0.45 litres of air in the capacity flask, as opposed to the maybe 10 cc of air in the aneroid capsules of an Altimeter; a factor of x 45 more possible sensitivity.
Yesterday when it was very blustery outside, my vario was moving about even in the house, in tune with the gusts of wind.


I suppose I must add my 2p worth to the Power + Attitude = Performance mantra... This is pretty meaningless unless you can supply figures with the equation, and I don't think you can do that, as the units are not comparable. Also as AF447 found out, it does not work for all attitudes.. i.e. tail first.


Just to prove it is so much drivel, lets put some numbers into that formula...Cessna 172.... 140 bhp + 4 deg nose up attitude = 144 performance units
then 140 bhp + 2 deg n/u = 142 units.... ??????? qed.
.

[Gertrude the Wombat]

Also as AF447 found out, it does not work for all attitudes.. i.e. tail first.
The point about AF447 was, surely, that power + attitude = performance would have worked, if only the pilots had known about it.

[scifi]

I think you will find that AF447 had cruise power set and nose up attitude held by the co-pilot.
What they overlooked was that their Angle of Attack, AoA was in the region of 40 to 50 degrees, for their whole descent.
.

[Heston]

Power + attitude = performance isn't meant to be a mathematical equation for goodness sake. It's part of how students are taught to understand that to make an aircraft go faster you have to do more than open the throttle (for example).
And AoA has very little to do with attitude. Anyone trying to fly without understanding AoA properly makes a poor job of it.

[Fl1ingfrog]

If the 1st and 2nd officers of AF447 had understood P+A=Perf. they would certainly be here today.

There were many factors that lead to that disaster. Other than fighter jets very few aircraft are fitted with A.of A meters. A Primary Flight Display (PFD) is able to be overloaded with a lot information including stall information, but can be set by the pilot by selecting the particular phase of flight required; the performance target required is then displayed. This can and does distract from the very simple act of cross reference to the horizon bar and speed. The experienced captain understands this and will always cross reference. However, the captain of AF447 was in bed.

The pilots were anxious and did not want to be flying in the conditions they found themselves. In turbulent sinking air they attempted to maintain altitude by overriding the auto pilot and pulling up which may have been stimulated by the audible warnings "pull up, pull up"; common in many large aircraft when the sink rate is outside certain parameters. A reference to the airspeed would have told them, in the first instance, all was well and at 34,000ft they had plenty of time to think first and then deal with any height loss.

scifi, the purpose of the "flask" is to dampen short term erratic indications. If you have gusty conditions in the house then there is no vario which will be of any use, I would recommend double glazing.

[LastStandards]

Hi D.D... you are quite correct the VSI is a very sensitive instrument. The most likely reason is that it uses 0.45 litres of air in the capacity flask, as opposed to the maybe 10 cc of air in the aneroid capsules of an Altimeter; a factor of x 45 more possible sensitivity.
Yesterday when it was very blustery outside, my vario was moving about even in the house, in tune with the gusts of wind.

Not quite a straight comparison - a light aircraft VSI and a vario are subtly different. A VSI works on the principle of lag, ie showing the difference between the current static pressure and the static pressure some seconds ago. Depending on the manufacturer this may be a lag of 4-8 seconds. They are therefore reasonably useless at showing instantaneous changes, but OK at showing a steady state eg continuous climbs/descents. Most useful in a light aircraft during instrument approaches when visual cues are lacking.

[Gertrude the Wombat]

Most useful in a light aircraft during instrument approaches when visual cues are lacking.
... and glassy water approaches, for the same reason ...

[wiggy]

If the 1st and 2nd officers of AF447 had understood P+A=Perf. they would certainly be here today.

Well like Godwin’s Law we end up here again, that said the above is almost certainly true, but....

The pilots were anxious and did not want to be flying in the conditions they found themselves. In turbulent sinking air they attempted to maintain altitude by overriding the auto pilot and pulling up which may have been stimulated by the audible warnings "pull up, pull up"; common in many large aircraft when the sink rate is outside certain parameters. A reference to the airspeed would have told them, in the first instance, all was well and at 34,000ft they had plenty of time to think first and then deal with any height loss.

You might want to read up on warning systems, the AF447 accident report and the CVR transcript again.

Now back to the thread.

[Big Pistons Forever]

For the private pilot course I did not teach instructor instrument flying until just before the solo cross county. This was to maintain the importance of looking out the windshield and using the natural horizon as the pilots solo reference for determining the attitude of the aircraft.

Since there was a possibility of getting stuck in low vis or worse in cloud during the solo cross country I wanted to make sure the student had the skills to escape.

Any instructor who does a pre solo lesson in weather that does not allow full control of the aircraft by visual means alone is IMO doing the student a grave dis-service.

[Fl1ingfrog]

wiggy, thanks for the heads up, apologise to everyone.

Having re-read the BEA report, I should have referred to the C Chord altitude alarm and not as I did the ground proximity warning. I still don't like the term "pull up, pull up" in any circumstance though. Works with auto-throttles but again can cause a mishap if the auto throttle is dis-armed.