For those in an aircraft approved for spinning the RoD will typically be close to 8,000~10,000 fpm – might not exactly be PAN-OPS.
I only re-entered for the edification of Airborne Again!….ie it is not just the pressure difference but the ratio of absolute pressure difference (which I expressed as a percentage)
Thanks, but I still don’t see that. The discomfort is caused by forces acting on the tissues. The forces are directly proportional to the absolute pressure differential.
The pressure differential is dependent on the speed of pressure equalisation which is also dependent on the absolute pressure differential between the ends of the Eustachian tubes. I.e. the pressure differential between the middle ear and the outside will rise to a level where equalisation happens at the same speed as the increase of external pressure due to the descent of the aircraft.
So I don’t see where the ratios enter into the picture.
Yes, the 4-seat diesel DA-40 will indeed climb to FL 180 on only 135 HP. However, it is not certified for that. It has an altitude limitation (yes, it is the chapter 2 LIMITATIONS section of the POH) of 16.400 ft.
When I first noticed that, I assumed the limitation was due to the engine control units not having been tested to higher altitudes. But the same limitation applies to the Lycoming-powered DA-40, so that explanation was not likely.
The only sensible reason I can think of for the altitude limitation is flutter. Flutter is a function of true airspeed, but VNE is given as an indicated airspeed. If the Vne of 178 KIAS is dictated by flutter margins at 16.400 ft, this margin will diminish above that altitude, flying at Vne.
So I guess that whenever flying above 16.400 ft, one should restrict max IAS to slightly below Vne – just in case the flutter theory is right :-)
I was always led to believe that the majority of aircraft are not limited by flutter and I think all part 23 and 25 aircraft are tested to maximum flight design speed (Vdf I think) which is higher than Vne. I think I remember reading somewhere (sorry I can’t reference as it was a long time ago) that some light and ultralight aircraft are more susceptible to flutter, but where this causes a limiting ‘TAS bust’ for lack of better wording then the Vne would be detailed to vary with altitude with at least a sea level and ceiling Vne given in the limitations section.
My opinion (they come free, as you know 
) is that some ultralights may not have been properly tested for flutter, but instead had the test data forged. I know this is not a popular topic, because it smells of a “certified” aircraft owner being “anti” the ultralight option (which I am not – so long as people appreciate that 450kg is going to result in an awful lot more compromises than say 1200kg) but there have been too many scandals in this area.
I don’t think the Lyco powered DA40 had a turbo, so it could not have had a flutter limited operating ceiling – because the ceiling will be limited simply by a lack of engine power.
Incidentally I am pretty sure that a descent of say 3000ft-1000ft will feel worse on the inner ear than from 13000ft to 11000ft. On a descent today from FL170, some plastic bottles I have got squashed way more in the last 2000ft than higher up.
Pirho,
I share your belief that most aircraft are not Vne limited by flutter. But DA-40 is an aircraft type designed by a company which historically has built a large number of motorgliders that definitely are Vne restricted by flutter – before starting in the the powered aircraft business. Some of the design philosofi is definitely carried over from TMG’s to powered aircraft like the DA-40 and that is why I believe that aircraft could be one of the types limited by flutter.
Yes, I also agree that all aircraft (Part 23 as well as Part 22 – gliders and TMGs) are tested to the design dive speed (Vd) which I recall as 1,1 times Vne – but that is irrespective of whether flutter or aerodynamic forces are limiting.
Thanks for the added info!
I still would have thought that if the type was limited by flutter, it would make much more sense to say in the POH that (made up numbers):
Ceiling = 20000
VNE @ S.L = 210KIAS
VNE @ 18000 = 200KIAS
instead of:
Maximum altitude = 16000
VNE = 220KIAS
Then again, we don’t know if that is in fact the true reason, and I am also not a designer/aerodynamics expert!
When I first noticed that, I assumed the limitation was due to the engine control units not having been tested to higher altitudes. But the same limitation applies to the Lycoming-powered DA-40, so that explanation was not likely.
Will that turbo-diesel even start at 16000 feet if it accidentally stops due to some finger problems or something else?
Thanks, but I still don’t see that. The discomfort is caused by forces acting on the tissues. The forces are directly proportional to the absolute pressure differential.
I don’t see that either – FWIW my aviation medicine textbook seems to think it’s the absolute pressure differential that’s important though it isn’t absolutely explicit. It also states that the effect is more pronounced at low altitudes. Fundamentals of Aerospace Medicine second edition Roy DeHart. pg. 576. I’m not sure what the ‘ratio of absolute pressure difference’ refers to either.
It is true that the pressure difference can be corrected by swelling, haemorrhage and leaking (‘transudate’) of the vessels in the middle ear, so in that case the volume needing to be compensated will influence the duration of any pain.
~~
Let’s take two models of the inner ear:
The ‘standard’ textbook model:
1) rigid walls, airtight with Eustachian tube blocked; pain determined by tension over tympanic membrane.
What I take to be AnthonyQ’s model:
2) compliant middle ear that can change volume; pain determined by process of compensating for any change in volume e.g. leaking of transudate into the middle ear.
In practice, both models are probably to some extent correct. In the 2nd model, the amount of pain you’d get would be identical at 9-10k feet or 2-3k because the volume needing to be compensated is the same. I still don’t see how AnthonyQ’s arguing the problem would be worse at altitude. In the 1st model, the pain would be worse at low altitudes because the absolute pressure difference is going to be bigger.
Something the textbook mentions that I hadn’t considered is that using oxygen can make the problem worse, because if you stop using oxygen you end up with a middle ear space full of O2, but it will be absorbed back into the body worsening the negative relative pressure across the eardrum. So this is one issue that could cause the problem to be worse at altitude (because when you descend you’re likely to use less).
But 1) the textbook says and 2) lots of people are reporting experiences to suggest that barotitis is generally worse at low altitude, given an identical descent rate.
I still don’t see how AnthonyQ’s arguing the problem would be worse at altitude. In the 1st model, the pain would be worse at low altitudes because the absolute pressure difference is going to be bigger.
In the end I actually agreed that since the ratio of absolute pressure change does not vary significantly in the atmosphere (in the range of altitudes we are talking about), there should be no perceptible difference in discomfort. This is largely due to the decreasing density of air with altitude (ie it is compressible)…as KWIF pointed out.
However I also pointed out, for academic interest, that in water (being of more or less constant density) the effect of changing depths at different depths, is significant even though the delta P may be the same…the point being that it is the ratio of change that counts….just that in the atmosphere you don’t really see it because the ratio is more or less constant…(again for the range of altitudes we are talking about)
This is nothing more than high school physics!
