The Aeromedical Adaptation of NASA’s experimental XV-15 Tiltrotor Research Aircraft — Project Proposal Submitted by Stanford Life Flight and Stanford Center for Design Research by Eric E. Sabelman, PhD and Ahimsa Porter Sumchai, MD
Prepared for Stanford Medicine Alumni Day April 22, 2023
The Bell XV-15 was an experimental American vertical lift tiltrotor aircraft. It was the second successful model and the first to demonstrate tiltrotor high speed performance capabilities relative to conventional helicopters. The XV-15 program launched in 1971 at NASA Ames Research Center in California. R&D contracts were issued to Bell and Boeing Vertol on October 20, 1972.
Eric Sabelman, PhD served for 20 years as clinical bioengineer and director of the Palo Alto Veterans Administrations Rehabilitation, Research & Development Center. Sabelman received a PhD in Bioengineering from Stanford University in 1976 following completion of masters work with NASA in spacecraft hardware in 1971. Sabelman was a NASA-Ames Research Center Postdoctoral fellow in space life sciences in 1979.
The Aeromedical Mission
“Born out of war. Air ambulance companies got their start in the USA after doctors in the Vietnam War saw how many lives were saved by airlifting wounded troops to well-equipped hospitals. The first hospital based helicopter transport program began in Denver in 1972. The number of programs more than tripled from 1981 to 1986.” USA Today July 18, 2005
By 2005, approximately 750 helicopters operated as air ambulances according to the Association of Air Medical Services. Aeromedical helicopters transport over 400,000 patients a year. Specialized teams transporting patients from the point of injury offer a 98% survival rate according to U.S. Air Force Aeromedical Evacuation.
In the 1980’s the National Transportation Safety Board found a series of air ambulance accidents were preventable. Tighter standards, better training and more stringent government oversight led to an overall decline in air ambulance accidents. According to the NTSB helicopters crash at a higher rate than airplanes. The most common cause of fatal helicopter crashes is flying in adverse weather. Research shows 91% of aeromedical helicopter crashes occur enroute to an emergency response and 30% occur at night. The incidence of medical helicopter crashes in the US decreased by 40% from 2009 to 2014 due to improved safety measures. [The Most Surprising Medical Helicopter Crash Statistics And Trends in 2023 — https://blog.gitnux.com/medical-helicopter-crash-statistics/]
Until now there has been no aircraft of domestic manufacture designed and customized to meet the requirements of aeromedical patient transport. Short — range first-response aircraft are typically helicopters with limited internal space and lift capacity, high noise and internal vibration and high maintenance and operating costs.
Fixed wing aircraft for longer range aeromedical transport can only land at airports, have poor access for loading patient litters and are incapable of adaptation for search and rescue missions.
The time is ripe for beginning a new effort to design and ultimately build an advanced aeromedical transport aircraft using current technology for materials, propulsion and flight instrumentation. This effort cannot be solely the province of the traditional aircraft designer, however, the needs and new capabilities of the medical community are the driving factors underlying this proposal. Bringing the requirements of the vulnerable “passenger” into the earliest stage of design of the candidate vehicle along with the ability of the engineering designer to incorporate the input and feedback of transport teams can be put to the test.
Improving the human factors aspects of the process of designing a mission — specific vehicle is a primary goal of this proposal. Most often, a vehicle intended for general purpose use is adapted post facto to meet specialized needs. The present project provides an opportunity to retain generalized aspects while building in specialized components at the earliest stages of the design process.
Stabilization and Transport of the Spine Injured Patient
“Safe movement of the human body is a primary concern for the high level spinal cord injured patient from the time of acute injury throughout his or her lifetime. A Stabilization Transport System with integral backboard and constant force traction has been developed to reduce the risk of further injury during helicopter transport to a specialized spine center and during routine nursing, therapy and diagnosis.” Sabelman et. al. Patient Handling: Positioning and Transfer of Spinal Cord Injured Patients
The Stanford Life Flight team flys with a stabilization transport system developed by Sabelman in association with Conal Wilmot, MD — Director of the Regional Spinal Care Injury Center at the Santa Clara Valley Medical Center.
The principal goal is to achieve the best aircraft for the aeromedical transport mission, building in the needs of the patient, medical crew, pilot and operating agency. While the outcome of the study may well be a recommendation to develop an advanced aeromedical transport using tiltrotor concepts or TAT, defining characteristics of an advanced aeromedical transport must satisfy constraints derived from five principle areas of definition:
A. The aeromedical market
B. Characteristics of the carrier
C. Identification and incorporation of advanced instruments and avionics
D. Conceptualization and design of an advanced medical retrofit
E. Ensuring FAA, NASA and NTSD approval of proposed designs
The journal Hospital Aviation annually surveys the nations aeromedical transport programs that are primarily hospital based. The average mission distance for rotorcraft aeromedical transports is 126 miles round trip. The average mission distance for fixed wing transports is 492 miles round trip. An advanced aeromedical transport must address the following missions defined by the needs of existing aeromedical transport programs:
- Emergency accident scene response
- Air search and rescue with first responder capability
- Neonatal transports with specialized equipment needs
- Cardiac and intensive care unit transports with specialized life support and equipment needs
- Head and spine injured tertiary care transports with advanced airway and spinal stabilization requirements
- Organ donor and transport of replantation teams for organ harvesting and preservation capabilities.
NASA’s Planning Group for Public Service Helicopter Technology determined in 1983 that a properly designed and equipped EMS helicopter can:
- Reduce accident response time by 30 to 80%
- Reduce mortality by 50% in both trauma victims and high risk neonates
- Transport patients to tertiary treatment centers consistent with their needs
- Provide advanced EMS service and transport to inaccessible regions where none is available
- Cargo hoist for air search and rescue
- Use skids, wheels or floats to land on unprepared surfaces and water
The Executive Summary of the Emergency Service Rotorcraft Technology Workshop [Washington, D.C. Oct 14–15, 1981 concludes the key technological needs for the EMS helicopter of the future are:
- Ride quality of a fixed wing aircraft, ie: reduced noise and vibration
- No tail rotor — reduced risk of injury during hot loading at emergency scene calls
- Small rotor diameter
- Improved cockpit visibility
- Improved crashworthiness
- IFR capability
- More affordable to purchase and maintain
- More reliable and dependable for emergency use
- More fuel efficient
- Specialized cabinet interiors to stock and shelve advanced medical diagnostic and communication equipment
An advanced aeromedical transport using the proven technology of the XV-15 tiltrotor research aircraft or TAT offers several intrinsic advantages over conventional helicopters used in this service. These advantages originate in the combination of vertical flight with the characteristics of fixed wing aircraft during forward flight. A medically configured tiltrotor design would incorporate all user tasks including first response capabilities. The design effort will concentrate on adaptation of the fuselage, including internal dimensions, crew placement, ingress and egress for patient and crew and location of medically significant facilities. The XV-15 is sized to have the minimum lift capacity and the maximum rotor area — landing footprint- desirable for foreseeable aeromedical missions of the future.