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.

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.

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.

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.

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.

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.
.

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.

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.

Seat of Pants vs. Vario / VSI
 

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.

Glassy water landings ...

PelicanSquawk wrote: 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.

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.

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.
.

The point about AF447 was, surely, that power + attitude = performance would have worked, if only the pilots had known about it.

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.
.

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.

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.

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.

... and glassy water approaches, for the same reason ...

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

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

Now back to the thread.