Artificial horizon Vs VSI instrument flying straight and level
 

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.

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.

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.

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!

TSRA

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

A Formula Flight
 

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.

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.

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

The little airplane is adjustable
 

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

B2N2,

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

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.

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.

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.

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

How does that work for you in cloud?


Gf

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

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.

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.

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.

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