Business & Commercial Aviation
The Lift Reserve Indicator
...instruments alone do not prevent accidents. They help, but it is the pilot's interpretation of the instruments, his knowledge of their limitations and his application of both that will prevent mistakes. In addition, we believe that the safety-consciousness of the pilots who have bought the LRI contributes as much to their safety record as does the instrument itself.
Nevertheless, we believe can be very useful - even critical at times - in preventing airspeed control accidents, particularly when maximum glide range may be essential and when maximum safe angle of climbs are suddenly required (such as recovering from wind shear-induced turbulence and airspeed fluctuations or avoiding obstacles). In addition, the LRI can indicate maximum endurance thrust setting.
In a nutshell, the LRI is designed to provide automatically, continuously and instantaneously a simple and direct read-out of an important aerodynamic state (L/Dmax) under all flyable circumstances, regardless of the aircraft's wing loading, center of gravity, flap configuration, density altitude...and a myriad of other conditions affecting aircraft performance.
There is essentially no interpretation necessary. The pilot only has to look at the indicator's pointer at the three o'clock position by whatever power and attitude setting is needed. In this way the pilot is assured that he is getting at least the minimum thrust to maintain flight, according to Huntington. All thrust available over and beyond this minimum is instantly available for climb.
...as long as the LRI needle is kept out of the red sector in slow flight and kept out of the red and blue sectors in turbulence, no airspeed control accident can occur. No knowledge of the prevailing circumstances and no knowledge of "book" approximations to the airspeed indicator are required.
To obtain similar performance using the aircraft's ASI a pilot would first have to know all the pertinent flight conditions, find the appropriate airspeed indication by consulting the aircraft flight manual and then fly that specific number. The rub is, however, that since the conditions needed to determine the proper airspeed probably will be changing continuously, so will the correct airspeed.
He also claims that the LRI is the only instrument that shows the exact maximum safe angle of climb within available engine output. Huntington contends that this feature could prevent low-level wind shear accidents in all size aircraft.
...The LRI comprises a simple three-inch, color-coded indicator, and a square-ended probe installed under the wing (on singles) or on the nose (of twins). The probe is mounted at such an angle and position that it protrudes approximately four inches into the free flow airstream and retains a constant relationship to the line of flight throughout a vertical arc of some 18 degrees.
Differential pressure sensed between two ports drilled in the forward edges of the probe is supplied through nylon tubing and drives the indicator needle on the cockpit gauge...the LRI gauge has red, white and blue arcs and L/Dmax speed is indicated when the needle points to the three-o'clock position - the juncture of the red and white arcs. Total system weight is about two pounds.
Our evaluation flight did indeed demonstrate the ability of the LRI to indicate maximum safe climb and glide angles, (including landing approaches) and safe level flight at just enough power to prevent sink. The flight also showed us that the LRI pointer would exceed the three-o'clock position where you would usually climb at best rate (for optimum deck angle) and cruise at some higher power setting than the minimum thrust needed for level flight.