Interviews with D P Davies on certificating aircraft

[CV880]

Hi Air Clues. I worked for a 747 operator that had to follow UK CAA air test requirements. My recollection is that Boeing were horrified to find out (rather late in the day) that airlines were stalling 747's and particularly were concerned about the loads on the horizontal tail due to the buffeting. IIRC, once Boeing found out people were stalling the 747 they required a Severe Turbulence Inspection of the tail after a full stall. After this the CAA changed the requirement to check stick shaker activation speed only.

[megan]

airlines were stalling 747's and particularly were concerned about the loads on the horizontal tail due to the buffeting

As a DC-10 crew proved, bits fell off. From Wiki,

November 11, 1979 – an Aeromexico DC-10 entered a sustained stall while climbing through 29,800 ft, to its assigned cruise altitude of 31,000 ft, over Luxembourg, Europe. The flight crew failed to monitor their flight instruments, so they did not immediately recognize that the plane was in a stalled condition. Instead, they blamed the heavy buffeting on the #3 engine, which they shut down, while continuing to hold the nose up. The plane continued to descend for one minute in a fully stalled condition, until the pilots lowered the nose and began a proper stall recovery procedure, which was completed at 18,900 ft. The #3 engine was then restarted, the declaration of emergency was canceled and the flight continued to Miami, Florida. Ground inspection revealed four feet missing from each of the outboard elevator tips, including the balance weights. The NTSB concluded that the sustained stall buffeting produced a dynamic structural overload on the elevator, which resulted in the failure of the elevator tips and balance weights. Further, the NTSB concluded that the autopilot had improperly been placed in vertical speed mode. That forced the AP to keep increasing the angle of attack to maintain the preselected vertical speed number, because maximum available engine thrust declined (normally) with increasing altitude. That caused the airspeed to fall below the stall speed of the aircraft

[jimjim1]

Just a note to thank the original poster Bergerie1 for posting these links and the Mod for making it a sticky on Tech Log. Very interesting. Just listened to them all back to back.

In my view the RAeS could do with a curator to improve the accessibility to this amazing material. For example compile appropriate playlists and or indexes.

[binbrook]

I've just seen #42. Reminds me of a student in a JP3 climbing through cloud in the Vale of York on a fine summer afternoon way back when. The alert QFI noticed that the speed was low and 'admonished' the student, whose reply was "You can either have climbing speed or a climb". Cue smart RTB, where they found both intakes were full of slush (well, pretty full anyway).

[Chris Scott]

Originally Posted by Meikleour

IIRC the 707-436 had a parallel yaw damper which had to be disengaged for take-off and landing and even with the extra fin area (main + ventral) their was a continuous mild tendency to dutch roll. This was not the case with the -321C which had a series yaw damper and thus no ventral fin. It would have been interesting to hear DPD's opinion of the stab trim motor stalling under large mistrimmed conditions.

Further to that, and the interesting response by Alan Baker, it may be worth mentioning that some of the early -320Cs may have retained the parallel yaw damper, while having the taller fin and no ventral fin. British Caledonian obtained a -373C from Britannia Airways (registered G-AYSI) that had been manufactured in 1963 and first operated, I believe, by World Airways. AFAIK it was the only BCAL -320C that was not designated as "Advanced". All our "Advanced" a/c had the serial yaw-damper. In my experience, G-AYSI had a tendency to Dutch-roll slightly in the cruise with the parallel yaw-damper engaged.

It may be worth mentioning that full-span (except at the pylons) L/E flaps on B707s are of the Krueger type - unlike the VC10s, all of which have full-span L/E slats.

[DH106]

Originally Posted by Chris Scott

It may be worth mentioning that full-span (except at the pylons) L/E flaps on B707s are of the Krueger type - unlike the VC10s, all of which have full-span L/E slats.

True - you & I would call them Krueger flaps after the way they deploy. Boeing seems to refer to them as 'slats' however if they maintain a small gap between themselves and the wing LE and thus function in the same way as a fully deployed slat - by accelerating air through the small gap. The 747 has what we'd call Krueger's full span, but I believe Boeing refers to them as Krueger's inboard and slats outboard.

[Chris Scott]

Originally Posted by DH106

True - you & I would call them Krueger flaps after the way they deploy. Boeing seems to refer to them as 'slats' however if they maintain a small gap between themselves and the wing LE and thus function in the same way as a fully deployed slat - by accelerating air through the small gap. The 747 has what we'd call Krueger's full span, but I believe Boeing refers to them as Krueger's inboard and slats outboard.

Yes, you are quite right to point out that it's the rather curious method of deployment that defines them as Kruegers, rather than the configuration when extended. You wouldn't want them to get stuck half way out, although presumably they would blow back in the event of hydraulic failure in transit?

Perhaps the slotted Kruegers should be referred to as "Krueger slats"? I can't remember if part of the L/E Kruegers on the 707-320Cs were "slotted" or not. No doubt it makes sense to slot the outboard sections only, as you describe on the B747, to help ensure the outboard part of the wing doesn't stall first.

The 707-320C may have had rather old-fashioned L/E devices but, IIRC, its T/E flaps were double slotted, whereas the VC10's were single-slotted. And are the B747's mainly triple-slotted, except the 747SP?

[Chris Scott]

Originally Posted by djfsheehan

Listened to the D.P. Davies audios last week just as the 737 Max groundings took effect. His comments made back in '92 on Boeings rather looser view on acceptable stall characteristics and the Boeing / FAA coziness around certification seem remarkably prescient ! In fairness to Boeing he also praises them in many other ways.

Yes indeed. Having finally listened to the four podcasts, one can see at least one reason why the RAeS might have waited 25 years before making them available to all and sundry. Davies's overall praise for Boeing is qualified only by his forthright criticism, as you point out, of its and other manufacturers' relationship with the FAA, particularly in respect of B727 and DC-10 certification. The original UK certification of the B737, OTOH, seems to have gone so smoothly that he doesn't describe any of it. One wonders what he would have made of the more recent developments of the type, particularly the expedient used to mount the LEAP engines in the MAX.

He was clearly unimpressed with the performance, late in his career, of non-pilot colleagues in reversing his original insistence, years earlier, that a stall-ident/stick-pusher be fitted to the UK-certificated B727-100, just as had been done many years earlier for the Trident/VC10/One-Eleven.

On a personal note, his comparison of the relative degrees of difficulty of flying the B707 and VC10 resonated. And his description of the pre-stall buffet on the VC10 took me back to an air-test in which the stick-pusher simply would not operate. Disappointing for me - and perhaps for him - that he never tested an Airbus type.

[daelight]

This really is a must listen. The warning signs were there but the pen pushers have made a dogs dinner of aviation unfortunately. Its only for the dilligence of the engineers, ground and air crews that have us flying as safely as we can given the circumstances. Those in the ivory towers have long lost the respect of real hard working people.