Dear Professor Orr

 

Introduction:

I am an aviation safety journalist and member of IASA. I write safety-oriented articles on behalf of IASA for Air Safety Week (ASW), an aviation safety weekly newsletter published by Access Intelligence of Maryland USA. I have a basal concept for significantly increasing air safety via a novel use of lasers. I would like to run that proposition past you for an appraisal of its technical feasibility. I have attached copies of the ASW's that have made passing mention of the potential application over the past few years. I shall describe the background for the requirement before getting into just how I see the application of laser technology potentially working to mitigate this hazard. There is an added potential air safety "bonus" application, for the same basic apparatus, which is also worthy of mention. My only personal exposure to laser technology in an academic environment was during visits to WRELADS at DRCS (now DSTO) Salisbury South Australia (the old Weapons Research Establishment -WRE) during a tour as a P3 Orion pilot (and later to the same laser laboratory during an RAAF Staff College visit). At that time (early 1980's) they were working on Laser mapping, Laser sonar techniques, Laser Target designation and Laser Range-finders. During my time at RAAF Edinburgh I was the Officer Commanding's representative responsible (as an instrument rating examiner and Flight Safety Officer) for overseeing the qualifications and currency of MOD(UK) Trials Pilots at Woomera and Evett's Field. I was also, as an airborne Range Safety Officer and Forward Air Controller, responsible for running firepower demonstrations and Fighter-pilot training utilizing laser-guided weapons at the Air Weapons ranges at Singleton, Puckapunyal, Rockhampton, High Range Townsville and Woomera. I also participated in early night vision goggles and laser specular diffraction trials. However my technical knowledge of the capabilities of different types of lasers is limited.

 

Background:

Some years ago, in what was known as the Roselawn accident (link + link), a French made ATR72 (Aérospatiale/Aeritalia) went down in severe icing conditions at Roselawn Indiana in the USA, killing all 68 on board. There have been a number of similar accidents to turboprop aircraft of that (and other) type(s). Turboprop aircraft fly at levels where they are susceptible to rain-ice (aka freezing rain and SLD or super-cooled large droplets). However most turboprops are protected from wing icing only by a very much "grandfathered" methodology - mainly due to a lack of any better concepts. The wing (and sometimes tail) incorporates a leading-edge rubberized inflatable glove called a de-icer boot. This boot is pneumatically cycled (normally via turbine engine bleed air) and causes thinly-layered leading edge ice build-ups to break up and be shed. This was an obsolescent technology when it was featured on the Dakota DC-3/military C-47 in the late 1930's. Other aircraft might have heated leading edges and engine inlets and prop blades that are protected by inlaid electrical filaments or bleed-air heating. None of these measures work effectively once rain-ice starts to build up all over an airframe and particularly when ice forms aft of the wing area covered by the boot. The unique and fatal characteristic of freezing rain is that it hits and sticks, freezing in position (but with some finite airflow-induced runback). This runback makes the use of boots quite drag-producing in that it permits spanwise excrescences to form behind the boot as ridges across the airfoil, from wingtip to wingroot. Additionally, because each turboprop engine's props rotate in the same direction, the airflow rotation causes a very different spanwise distribution of the icing deposits to build up on the port side (as opposed to the starboard side) of the aircraft (fuselage, empennage and most importantly, its wings). This asymmetric spanwise distribution of ice build-ups causes vastly different stalling characteristics between the two wings. The asymmetry ultimately leads to an unrecoverable unstable spiral (as a prelude to most typical icing accidents). In this typical accident (for instance) (link), the aircraft very quickly "departed" whilst cruising on autopilot and only shortly after entering icing conditions. A "departure" in this context is an uncommanded roll that's usually entered once an autopilot is no longer able to hold (and therefore conceal) the increasing asymmetry of lift and drag.

 

The Problem

The accumulation of rain-ice, day or night, can be very insidious. In the final stages of accumulation the rate of deposition increases (because of freezing precipitation impacting upon existing ice) and the rate of accretion can increase significantly. It is an insidious onset. This is because in some aircraft auto-throttle mechanisms can increase power to maintain scheduled airspeed and autopilot pitch auto-trim can progressively trim back on the elevator trim to disguise a slow performance loss due to the increasing overall drag and weight increments. Ultimately, in a typical accident, one wing will reach its "new" stalling angle and the aircraft will roll uncontrollably into an unrecoverable spiral. The "new" stalling angle will be reached at a higher speed and lower angle of attack because of the wing's ice-degraded aerodynamics. Prop rotation direction seems to decree that most departure rolls are to the right (i.e. the RH wing stalls first). This is the aspect that causes most surprise to pilots - a very premature stalling autorotation entry (aka spin) at a much higher than normal airspeed. Because the instinctive reaction of a pilot is to oppose an uncommanded roll with aileron, this only serves to embed that wing more deeply in its stalled condition. In some scenarios the pilot will not be aware of the need to first disconnect the autopilot prior to attempting recovery - or it will not self disconnect due to overload, further compounding a pilot's control loss conundrum. Here you will need to know that an aileron input meant to raise a dropping wing acts to increase the lift on that wing whilst decreasing the lift on the other wing. This normal rolling functionality of a differential frise aileron ceases once that one wing has stalled due to an asymmetric accumulation of ice-induced drag (and attendant loss of lift due to airfoil profile distortion). i.e. you cannot "increase" lift (to raise it) on a wing that has stalled. Freezing rain induces a flight control loss that is sometimes referred to (somewhat confusingly) as an aileron hinge-moment reversal caused by the ice-ridging. I prefer my explanation of the process however. It more aptly describes the causation.

 

The Federal Aviation Administration (US FAA) has been very slow to address any risk mitigation measure other than pilot education. However despite this, light jets, turboprops and IFR-rated reciprocating-engined twins continue to augur in due to unanticipated encounters with severe icing conditions. Legislating against operating in severe icing conditions is very much akin to shouting at the wind. It is very easy for pilots to enter even extreme icing conditions without noting the subtle change in the environmentals. Weather radar is of little assistance in avoiding areas of severe icing. The deadly scenario normally results from unknowingly entering a new airmass (typically the occluded warm front). In addition, aircraft frequently attempt to take off with what seems to be an insignificant layer of ice, only to find that the aircraft enters a stall soon after take-off (see link & link). Some super-critical airfoils are very intolerant of even a thin rough layer of frost. The answer may lie in utilizing laser technology to both detect and eliminate ice deposits, inflight and preflight (the latter measure as an alternative to the ecologically unfriendly liberal spraying of anti-icing fluids).

 

The Proposed Solution:

It may be possible to mount aft-facing faired cupolas incorporating a mapping laser profiler (to detect ice accretion) and a thermal laser de-icer..... one system below the nose (forward of the nose-gear housing) and one above the cockpit. Two such cupolas should enable sweep coverage of the wings (upper and lower to at least 2/3rds chord, the point at which laminar flow will separate from a clean wing and the boundary layer become turbulent). It could also cover tail surfaces including vertical fin (aka empennage), props, spinners, inner nacelle surfaces and engine intakes. More distant surfaces would be accommodated by a slower thermal laser "scan-rate" (or the same scan-rate and a power ramp-up). It is appreciated that there could be a power management problem however this would be associated with the thermal laser's required range and intensity. I expect that even in the most severe icing conditions the charge/discharge firing cycle of a heavy duty, pulse-power, air-cooled capacitor should be able to cope - as long as both engines (and both AC generators) of a twin-engined airplane were operational (brash assumption #1 [BA#1]). The typical accretion rates are such that wings should be able to be treated one at a time - however it might also be possible, via a CNC controller, for the left wing upper surface to be "treated" by the upper cupola while, at the same the lower mounted cupola is "treating" the right wing's lower surface. (BA#2). Thinking here of 4 axis CNC motion as utilized in complex laser welding. Nd:YAG pulsed lasers are assumed to have the ability sought. I am estimating a full duty cycle in the heaviest icing to be around 60 seconds ON and 60 secs OFF. The physical nature of a thermal laser "lance" might be that it's able to "peel" an icing layer (i.e. with the assistance of airflow) by merely melting a strip along the airfoil leading edge and then attacking any residual ridges further back on a return swipe - following that initial leading-edge clearance. (BA#3). It is assumed that the underlying surface (composite or metal) would be impervious to the effect of the thermal laser (BA#4). Additionally I am assuming (but uncertain) that the "normally unutilized" 65% to 75% of each generator's rated capacity (about 20 to 30KVA) would suffice to charge each cupola's capacitor in turn or indeed whether near-infrared solid-state lasers would be the technology of choice. Similarly I am unaware whether icing mensuration mapping of the airframe would/could take place simultaneously with a de-icing sweep...or utilize the same laser beam. That simultaneous coverage and treatment would seem logical for a graduated and measured appropriate response using a variable power approach. I am also unsure whether a single-engine emergency operation utilizing an APU (auxiliary power unit) as the 2nd generator would be feasible...... or whether single generator operation would be sufficient for the likely power requirements. It should be borne in mind here that single-engine instrument flight rules (IFR) airplanes such as the Pilatus PC12 and Cessna C208 Grand Caravan would also benefit greatly from this technology (if it proved to be feasible). The C208 in particular is causing great concern because of the number of icing related accidents (always fatal).

 

Adjunct Solutions (possibly utilizing the same technology):

a.  Some Norwegian airfields are now using taxi-through shelters for surround Infrared lamp heat clearance of ice in lieu of fluid boom-spray de-icing before take-off. It is a novel solution to an age-old problem, but at a great cost in terms of infrastructure outlay. It also has shortfall connotations in respect of holdover times (see below). The proposed integral onboard system (dubbed Laissez-faire....Laissez faire being French for "leave alone") should be able to be used in a similar fashion on the ground at the pre-takeoff holding point or whilst taxiing out - but with obvious laser safety range optical hazard considerations. For that purpose its laser mensuration of the "visible" airframe would have to be certified for establishing an aircraft to be sufficiently clear of icing contamination for take-off. It should be borne in mind that many pilots have in the past (and still do) try to save their companies the many thousands of dollars that it costs to de-ice a medium sized airplane. It's a risk-taking exercise that often backfires disastrously. In addition, there are ecological concerns with the many different types of de-icing fluids in use. Some aircraft have also experienced control restrictions and control freezing in flight due to coagulation of rehydrated de-icer fluids trapped in elevator and aileron hinge-line recesses. A further consideration is "hold-over time". That is the ground holding period beyond which an aircraft must taxi back in for a second de-icing. Fluid de-icing occurs at a great cost in terms of fuel, lost time, queuing and accidents. Integral ground de-icing would be a great economic boon to airlines, airports, air traffic control and it would benefit the environment.

 

b.  A slightly lesser hazard for airliners is the bird-strike hazard, particularly on take-off and below 10,000ft. In non-icing conditions when flocks of birds and geese are a seasonal threat, it might be technically feasible

    (i) if the cupolas could face forward and

    (ii) either alone (or in concert with a high PRF weather radar mode) detect any bird threats ahead (along or adjacent to the flight-path) and react by laser-zapping the avian intruder(s).

With the increasing number of twin-engined airliners taking advantage of long-range ETOPS dispensations for heavy twins, there are many airplanes that can ill afford to ingest flocks of birds into both engines. At the very least a close encounter of the avian kind usually means an expensive "dump-fuel and land-back" operation for damage checks. The psychology of birds, upon an encounter with a looming adversary or "shocking experience" is invariably to instinctively peel off and dive for speed. Zapped a few 100 meters ahead, that characteristic would normally put it below an airliner's projected climbing or approach flightpath. On the runway it might cause a bird to clear the ahead sector. [ETOPS = Extended Range Twin-Engine Operations]

 

c.  There are presently two different technologies vying for ascendancy in the field of countering MANPADS threats (shoulder-fired missilry aka man portable air defence systems). Northrop Grumman and British Aerospace are fielding onboard systems. The technology is once again one of detection and elimination (i.e. destroying, spoofing or decoying an incoming threat). Israel's Flightguard will eventually be replaced by a laser-based system. However it's not assured that any MANPADS threat would have to be fired from the ground (i.e. it could come from an overflying helo or light aircraft). As most of these missiles are infra-red homing rear-hemisphere heat-source homers, an onboard IR laser should be able to seduce a near miss if it was able to ramp up its proximate power and swamp the incoming missile's terminal guidance with an IR "bloom". It is realized that any expectation of making an anti-ice/de-ice purpose-designed system into an all-encompassing hemispheric panacea is asking for the moon and stars. However a laser specialist working toward his PhD in your institution (Macquarie Univ's Higher Degree Research Unit) could logically complete a thesis upon the implausibility (if not the impossibility) of filling each such requirement (or alternatively, what would be required to do so, and whether it could all be incorporated within an airliner's weight and space limitations).

 

Summary:

Passengers on regional turboprops and smaller bizjets do not realize the extent to which their safety is being compromised due to the inadequacy of anti-icing systems and the icing-prone cruising altitude band of those aircraft. To some extent the same statement might be true for aircrews as well. Larger jets have the bleed air capacity to generate sufficient de-ice and anti-icing capacity but might still benefit from the proposed technology in savings on proportionately higher ground de-icing costs. Because of the utilitarian nature of laser technology, it may be feasible to design an omnibus system that can also address the other threats described above. That would in part overcome the fact that the icing and bird-strike threats tend to be seasonal in nature for regional aircraft. The safety connotations and optical hazards of utilizing lasers for ground de-icing would need to be examined. The ability of regional turboprops to provide the power to meet the electrical loads is a major consideration.

 

Request:

In the near term it would be appreciated if you or your department could give a synopsis opinion of the practicability and feasibility of utilizing laser technology either now or in the future for airframe and engine/prop de-icing - and some idea of any foreseen limitations of such a system. In the medium to longer term you might be prepared to look into the adjunct applications and give an opinion on whether a laser-based inflight de-icing system could render alternative/secondary service in any/all of the other three roles described above.