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Crossunder
17th Jun 2006, 12:38
Hi all experts!

I just finished my rating on the Q400 and have a question about the "false" Vmo restriction imposed for flying below FL100. Our OM-B states the following limitations:

0 - 8.000ft 245KIAS
@10.000ft 282KIAS
@18.000ft 286KIAS
@20.000ft 275KIAS
@25.000ft 248KIAS
@27.000ft 238KIAS

Naturally, there have been quite a few Vmo exceedances during initial training for some of our crew, as they descend through 10.000ft and the Vmo suddenly drops almost 40 knots the next 2.000ft (which is highly illogical and unusual for any aircraft btw.). We're used to the -300 version, which does not have this problem, and we fly a mixed fleet.
From what I understand, DeHavilland has programmed the Vmo pointer to indicate a fictious limit, in order to prevent pilots from exceeding 250KIAS below FL100. This due to the wind shield not being able to cope with a bird strike at speeds above 250KIAS.
Now, the 250 knots limit below FL100 is an international speed restriction (?), mostly because of the danger of bird strike at altitudes below 10.000ft. But why impose a "false" restriction instead of just trusting the pilots' judgement on this? Other aircraft do nor have this limit built into their Vmo pointers. The "free speed" option is often given by ATC. I consider it acceptable to keep, say 300KIAS, below FL100 if the risk of bird strike is negligible/unlikely; e.g. birds don't fly at night or in clouds, so why the need for speed reduction?

Is it an FAA/JAA restriction, commonality issue with the DHC8-100/-200/-300, or something else?

Capt Fathom
17th Jun 2006, 12:58
birds don't fly at night or in clouds, so why the need for speed reduction?

I not sure that anyone has confirmed this ... with the birds. Why push your luck, it's not necessary!

knertius
17th Jun 2006, 13:02
birds donīt fly at night? tell that to the damn thing that impacted with my no.1 engine in varna at night a couple of months ago!
and if i remember correctly an austrian F70 had a birdstrike IN CLOUDS about 4 years ago while approaching CPH.
other than that i have to agree with you. quite an irritating feature. but, on the other hand, its often been free drinks for me back at the hotel because of the FO not reducing speed fast enough :)
cheers
k

Crossunder
17th Jun 2006, 13:31
OK, so maybe some fly at night, and others really do fly in clouds (I bet it was purely by mistake though...). My point was really that the restriction seems like a rather unnecessary "we don't thrust you pilot-twats to do this right"-statement.

What about a B737? Does its wind shields stop a bird from entering the flight deck at an effective speed of 300kias? I dunno... Just wonder WHY the restriction is there on the Q4, and not other aircraft. :8

Mad (Flt) Scientist
17th Jun 2006, 16:23
That's not a "false" Vmo restriction. It's real, and not there because we don't trust pilots to stick to some ICAO 'speed limit'; it's there because the aircraft could not be certified with a higher speed at those altitudes.

Birdstrike is a real cert requirement, we don't get the option of saying to the authorities "go on mate, let's not worry about birds, it's never gonna happen". Even if the "speed limit" you mention were totally waived, the Vmo would be unchanged. It's a design restriction, not operationally motivated at all.

Also, there are similar Vmo cutbacks on the CRJs (I don't have a chart to hand) which are also, IIRC, birdstrike-driven. (I think it's a cutback from about 335 to 300, but that's pure memory and could easily be wrong)

Finally, be aware that while the windshield may be the most critical birdstrike case, by increasing speed to 300kts, you're increasing the birdstrike energy by some 50% above the assumed certification level. At some point you're going to get to the point where OTHER strikes are going to cause damage - especially if you hit a bird above the certification mass.

Crossunder
17th Jun 2006, 17:20
Now we're getting somewhere! ;) So what you say is that Vmo is not just a margin to Vd or some sort of airframe buffet/flutter/undesirable handling characteristics etc? And if I understand you correctly, a B737, A319 or Lear Jet have wind shields that can withstand a bird strike at, say, 300KIAS? Then I wonder why the Q400 could not have been fitted with stronger windows? S'pose it would have been much more expensive? Or just that they use the same material as in the older series aircraft, which did not have the "luxury" of travelling at speeds greater than 250-ish anyway.
It would also be very interesting to have a look at the actual certification requirements. Is 8-10.000ft the official certifiation altitude limit when it comes to bird strike, or is it an altitude chosen by DeHavilland?
I'm just trying to understand the hows and whys, and get the full story behind this restriction. That way I can explain it to my colleagues as well, and thereby achieve compliance through knowledge. Or something to that effect ;) :ok:

ESSEX BOY
17th Jun 2006, 17:27
Citation Biz jets have the same restriction VMO jumps to 260kts below FL80/8000ft for the same reasons ... we stick to the same rule as Knertius ... whoever sets it off buys the beers :E Does have the worrying CRM implications that you won't say anything re speed if your gonna get a beer though :rolleyes:

Must be a bit of a pain in the Citation X as its the fastest commercial jet now and still limited to 260kts below 8000ft because of windshield protection.

EB :ok:

Mad (Flt) Scientist
17th Jun 2006, 19:25
With the usual caveat about the specific regs applicable to a given type, here is the relevant section for birdstrike....

Sec. 25.571 Damage--tolerance and fatigue evaluation of structure.
.....
(e) Damage-tolerance (discrete source) evaluation. The airplane must be
capable of successfully completing a flight during which likely structural
damage occurs as a result of--
(1) Impact with a 4-pound bird at Vc at sea level to 8,000 feet;

Vc is the annotation used for the structural design speed which is usually coincident with Vmo, although they can be different if so desired...

VMO/MMO must be established so that it is not greater than the
design cruising speed VC

So, to answer the question:

Is 8-10.000ft the official certifiation altitude limit when it comes to bird strike, or is it an altitude chosen by DeHavilland?

8,000ft is a specific certification requirement. The 'illogical' nature of the Vmo cutback isn't the fact that it has a given value at 8,000ft - it's the fact that the speed can suddenly increase at 8,001ft. Logic would suggest that some kind of ramped Vmo limit would be consistent with the (PRESUMED) declining probability of bird encounters between, say 8000FT and 12000FT. But, of course, the reg as written and applied has worked, so why mess with it I guess.

Now, I don't know the certification issues of the various types, but:

And if I understand you correctly, a B737, A319 or Lear Jet have wind shields that can withstand a bird strike at, say, 300KIAS? Then I wonder why the Q400 could not have been fitted with stronger windows?

Be very careful in assuming that because the Q400 has a lower limit, its windshield is less strong. There are many instances where, due to more stringent regs, or more stringent interpretation of regs, a later-certified aircraft may have limits applied that an older types does not - even if the "real" strength of the windshield is actually better for the more modern type. You can't reverse engineer the design characteristics from the limits without knowing a LOT of background ....

145qrh
17th Jun 2006, 20:58
Crossunder ,

The ERJ145 drops from 320kts at 10,000' to 250 kts at 8000', flight idle and v/s of roughly 700fpm seemed to work IIRC...Just keep the trend line reducing at he same rate as the red-line is reducing...:)

Should be easy with the 6 blade noise generators to get the speed back.

Capt Claret
17th Jun 2006, 22:18
A DH8 had an altercation approaching YBRM about 10 years ago. The eagle went through the fibreglass wing-root to fuselage fairing, taking out the wiring to the left (I think) engine, cracking the wing spar caps, and causing the spar to bend rearwards. Many, many months out of service.

If Mr Bombardier says max 250 < 10,000' then that's it.

Crossunder
18th Jun 2006, 09:20
Thanks very much for your answers, chaps! Very informative and to-the-point :ok:

Pimp Daddy
18th Jun 2006, 09:32
A DH8 had an altercation approaching YBRM about 10 years ago. The eagle went through the fibreglass wing-root to fuselage fairing,

Here's the report for that one, made quite a mess.

http://www.atsb.gov.au/publications/investigation_reports/1996/AAIR/pdf/aair199601590_001.pdf

AerocatS2A
18th Jun 2006, 15:55
A DH8 had an altercation approaching YBRM about 10 years ago. The eagle went through the fibreglass wing-root to fuselage fairing, taking out the wiring to the left (I think) engine, cracking the wing spar caps, and causing the spar to bend rearwards. Many, many months out of service.
Was that the one that suffered additional problems after it had been repaired? Something to do with the undercarriage?

Capt Claret
18th Jun 2006, 23:13
AerocatS2a,

Yep. I believe a bolt was inserted back to front and subsequently prevented the nose gear extending.

At the time there was a lot of pressure on engineers to get the machine back into the air. IMHO it can be considered another example of "more haste - less speed". :ugh:

Maude Charlee
25th Jun 2006, 12:37
Turning the question on its head, why on earth would you want to be tooling along at 300 KIAS below FL100 anyway?

You have to reduce to 210 KIAS at 12 nm anyway, so just what are you going to achieve by bashing around at high speed within the preceeding 20 miles or so?

Are you really so desperate to save 30 seconds on your block times?

You're just inviting trouble if you ask me.