To all:
I completed a risk analysis on high-speed flight below 10,000 feet for Transport Canada a couple of years ago. The analysis was commissioned out of concerns over the increasing large flocking bird population numbers in North America and the need to understand the safety risk vs. the economic benefits.
While researching the issues I obtained the following historical background on the 250 knots below 10,000 foot rule:
FAR 91.117 (a) states: Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 mph).
As reported in another post, the rule was established after a mid-air collision over Staten Island. However, there had also been two other mid-air collisions around that time period. The FAA originally considered implementing a rule restricting airspeed to 250 knots below 14,500 feet MSL but compromised at 10,000. In fact an early draft called for 160 knots at 14,500.
What is most fascinating is that from this point forward current airframe and engine rules were developed based on this regulatory speed requirement, using 250 kias as a test condition.
My research of the certification standards, the North American bird strike data and the potential efficiencies revealed that 60% of bird encounters above 3,000 feet AGL involved raptors or waterfowl. What is interesting is that as altitude increased the percentage increased. Waterfowl and raptors present a significant risk due to the fact that their mean weights exceed the engine and airframe certification standards for the majority of the current world fleet and for waterfowl these encounters will be with flocks – something that the certification standards do not contemplate. Couple this with the fact that of the birds in North America; 14 species with a mean weight above 8 lbs, 13 have an increasing population trend and for the 31 species with a mean weight between 4 and 8 lbs, 24 were increasing and 5 were stable, and we have significant increasing safety risk.
I then analyzed impact forces based on the FARs and compared the calculated impact force for various bird weights at different indicated airspeeds. The results are very sobering. Due to the fact that impact force increases as a function of the square of the TAS, a constant IAS climb leads to increasing impact forces. In fact if you hit a 4 lb bird at 300 KIAS at an altitude greater than 4,500 feet you will exceed the certified airframe strike impact force for many jet aircraft.
If one examines the potential system efficiencies of high speed operations below 10,000 feet there is no conclusive data to show that there are any meaningful increases in efficiency. The key here is to examine the total efficiency of the traffic system – if a number of aircraft are accelerated above 250 KIAS and ATC requires them to fly the same speed for traffic separation, a number of aircraft may gain slight efficiency advantages, but a number will also incur penalties. This can easily be seen by examining the range of ECON climb speeds that an FMS will generate if the speed limit is removed, they can vary from 265 – 315 KIAS on the narrow body airbus fleet (A319, A320, A321). The net result is that there is limited if any overall system efficiency increase.
When you consolidate all the above information; regulatory history, certification standards, bird strike data, bird species population data, impact forces and limited efficiency gains there is no reasonable safe case to allow high speed flight departures below 10,000 feet.
The preceding information is only an outline of the report. I will try and see if I can obtain from Transport Canada to post the entire report.
If any of you wish to read further information on bird strike issues please take the time to visit the Transport Canada website and download/read “Sharing the Skies” a compendium of bird strike information. It is an excellent resource and safety tool.
The URL is:
http://www.tc.gc.ca/CivilAviation/Ae...13549/menu.htm
Richard Sowden
A320 Captain and bird strike researcher