What Is the Lift Reserve Indicator?


The Lift Reserve Indicator has been called variously an angle of attack meter, a lift management system and an airspeed director. 


The Lift Reserve Indicator integrates both airspeed and angle of attack in a single readout reliably and continuously displaying an aircraft's margin over stall despite the wide range of variables to which an aircraft is subject.

For reasons that are embedded in the romance and history of aviation and in the freedom that flying evokes in all of us, exemplified by the expression "right stuff", we find that many pilots argue, and argue with conviction, that airspeed, stall warner, training and instinct are sufficient to meet all conditions from rotation to touchdown.

We respond, and NTSB accident statistics support, that miscalculation and loss of control are unfortunately significant contributory factors in many, many aviation accidents. We are all familiar with the term "pilot error". The LRI provides information that could help to eliminate inadvertent stall/spin, density altitude miscalculations, and unrecognized excursions into windshear conditions. The LRI also provides exact control necessary for all STOL conditions.

Far more than a 'stall warner', the Lift Reserve Indicator provides constant reference to a base line datum, zero lift reserve, which is established during calibration. The result is safety and control, critical during slow speed flight and during maneuvers at any speed where lift approaches zero. The LRI eliminates guesswork and calculations and is reliable under all conditions of weight, center of gravity, flap position, density altitude, turbulence and angle of bank.

The LRI consists of a rectangular, aerodynamically shaped airstream probe and a display gauge. The probe is mounted so that it is positioned in relatively undisturbed air and is effective on all types of General Aviation aircraft including singles, twins, float planes, pushers, ragwings and homebuilts. There are two air pressure ports, each on separate faces of the probe, which are piped to the instrument display. The differential pressure between the two ports yields lift reserve readouts.

 


The LRI display consists of a single arc divided into three sectors. The first is Red; the second is White; the third is Green.

The Red sector of the arc indicates that the airplane is no longer generating sufficient lift to sustain level flight. At the top of the Red sector, just below where it meets the White sector, the airplane is sinking. Sink is imperceptible. Deeper into the Red sector the airplane approaches full stall.

During takeoff when the LRI needle clears the Red sector and enters the White sector, the airplane has sufficient lift for takeoff.  


The White sector of the display is the slow speed region of the aircraft. It is the region just above zero lift reserve. During takeoff and climbout this sector of the display indicates

During approach and descent, flying the airplane in the White sector of the gauge allows the aircraft to safely dissipate the kinetic energy of lift producing the slowest approach and shortest rollout.

The Green/ Blue sector of the Lift Reserve Indicator is the area of "buckets of lift".




The following NTSB report contains an analysis of a recent General Aviation accident which caused three fatalities. Pilot error was a significant contributory factor.

Had there been a properly calibrated Lift Reserve Indicator on board, it would have alerted the pilot in command to the

A glare shield mounted LRI display would have been in the line of sight of the pilot in command.

A properly calibrated LRI would have instantly alerted the pilot in command to changing wind conditions and he would not have been dependent upon a slowly responding and fluctuating ASI.

With a properly calibrated LRI on board, the pilot in command would not "misjudge the margin of safety above the airplane's stall speed".

THE ACCIDENT SCENARIO: The statements provided by witnesses indicated that the airplane's climb rate and speed were slow and that after the airplane transitioned to an easterly heading, it rapidly rolled off on a wing and descended steeply to the ground in a near vertical flight path, consistent with a stall.  

Based on performance data provided by NASA, the Safety Board determined that the rainfall present at the time of takeoff could reduce the airplane's lift by as much as three percent, increasing the airplane's stall speed by about 1.5 percent.  

The Safety Board found that the pilot in command decided to turn right immediately after takeoff to avoid the nearby thunderstorm and heavy precipitation that would have been encountered on a straight out departure. Witness statements indicated a gradual turn, consistent with a bank angle of about 20 degrees. With the flaps set at 10 degrees, this turn would increase the stall speed about three miles per hour, from about 59 mph for steady level flight to about 62 mph.  

Because the airplane was about 96 pounds overweight at takeoff, the Safety Board found that this would have increased the stall speed another two percent. 

... Density altitude at the time of takeoff was calculated to have been 6,670 feet MSL. According to airplane performance data from Cessna, the high density altitude and the airplane's overweight condition would have decreased the airplane's best rate of climb speed from 84 mph to 81 mph, with a climb rate of 387 feet per minute. Thus, the airplane had decreased performance with an increased stall speed. However, it should have been able to climb and turn safely. The Safety Board analyzed possible reasons why this did not occur.  

Investigators believe the evidence shows that the pilot did not lean the fuel/air mixture for maximum power for the high density altitude takeoff. The mixture knob was found in the full rich position at the accident scene. Although it is possible that impact forces moved the knob forward to full rich, investigators noted that the linkage rod was not bent. Investigators also noted that the pilot did not stop at the end of the runway before the takeoff roll, which would have been the most common and appropriate time to adjust the fuel/air mixture.  

Carburetor icing conditions existed at the time of takeoff. Investigators noted that without the application of carburetor heat during taxi and runup, ice may have formed the carburetor and reduced the available power at takeoff. The carburetor heat control was found in the "off" position. The pilot's failure to stop at the end of the runway also suggested to investigators that he did not perform a pretakeoff checklist, which would have included a magneto check and check of the carburetor heat.  

flight visibility at the time of the stall was most likely substantially degraded due to precipitation, eliminating a visible horizon, the pilot in command could have maintained ground reference by looking out the side window. However, this could have been disorienting to the pilot because of the need to scan to his left to see the flight instruments in front of the trainee and to his right to see the ground as he attempted to operate the airplane at a low speed, with a lower than normal climb rate.  

The Safety Board found that the wind conditions would have made it more difficult for the pilot in command to maintain a constant airspeed and rate of climb and could have resulted in an unintended reduction in airspeed to below the airplane's stall speed. The wind conditions also may have affected the pilot's perception of the airplane's speed. What was initially a crosswind during the takeoff roll and initial climb, became a tailwind after the airplane began its right turn. Because the pilot was most likely looking outside during the special VFR departure, he may have not been adequately monitoring the airspeed indicator, or may have had difficulty monitoring it because of airspeed fluctuations, and may have mistaken the increase in ground speed as an increase in airspeed. This may have led him to misjudge the margin of safety above the airplane's stall speed.  

The Safety Board noted that the pilot in command's limited experience in operating out of high density altitude airports should have prompted him to be cautious, in addition to his knowledge of the storm that was moving in and the report of wind shear from the Cessna 414 pilot who had just departed. 


NOT TO BE USED AS A PRIMARY FLIGHT INSTRUMENT

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